WI: 7.62x51mm never becomes NATO standard

what advantage does keeping .30-06/.303 or moving to something like .280 brit/7.92* 41 have over 7.62*51?
Neither option really offers much, truth be told. The smaller calibers (6mm-6.5mm) will offer lighter cartridge weight (possibly also permitted a lighter rifle/weapon system) and increased control under automatic fire at the expense of "stopping power" similar to where NATO settled with the 5.54x45. It is because of this short coming that it may desirable or even required to keep a heavier .30+ caliber round for long range and heavy hitting requirements between that light infantry round and the 12.7x99mm (.50 BMG).

Regarding the .280, it also doesn't offer much in terms of universality that the 7.62x51mm doesn't also do, which is why I was advocating for a .270 caliber round which best balances the advantages of the smaller .22-.26 caliber rounds (light weight, low recoil, flat trajectories) and the .28-.32 caliber rounds (stopping power, momentum, resistance to drift/deflection, armor penetration at range, etc.). I guess, the summary is that if you are willing to accept a two caliber arsenal then take one from the lower end and one from the higher end--as NATO has had since the introduction of 5.54 to pair with the 7.62--where as if you are really serious about a "one-size fits all" universal round the best compromise is some in the 6.5mm-7mm range, with 6.8mm being the best balance of all. I was basing it on the premise of NATO not accepting 7.62x51mm as per the OP.
 

Deleted member 1487

On the other hand, if we are looking at adopting the .270 or .280 British (or similar 6.8mm-7mm round) then really we are accepting a compromise to behave as a universal round for use in the standard infantry rifles, assault rifles, and LMGs. In those circumstances the .270/6.8mm is probably the better choice. The .280 itself was a compromise proposed by the British to appease the US requirement for long range shooting (taken to the extreme of compromise with the .280/30). Modern evaluations and ballistics development have revealed that a longer and heavier .270 FMJ (in the 115gr - 130gr range, making for a very long bullet with high cross-sectional density) produces just about the perfect balance of ideal trajectory, stability, and wounding capability. As I stated in an earlier post, a mid-length .270 round--something like the wildcat .270-08, perhaps even a shade shorter like a 6.8x48mm (longer and more powerful than the modern 6.8 SPC) instead of 6.8x51mm--would probably be the best option as it allows a relatively short action for high rate of fire while also permitting 1000 yard accuracy equal or superior to the 7.62x51 NATO of OTL with the right load.
The issue with that is that while the 'ideal' round is good at 1000m, you're just making a better WW1 rifle caliber...which was basically already perfected with the Swedish 6.5mm round. Great for sniper rifles and MMGs (even 'heavy' mgs), but that only creates an overkill infantry hand weapon round. Because for infantry hand weapons in the era before optics could be placed on all infantry weapons, combat was largely limited to 300m with maybe 7% happening above that, because even if the terrain was flat you still couldn't really see someone in drag color clothing trying to make use of cover beyond that point. That's also before the smoke and other sight impediments of battle kick in. With combat at 300m or less then you've got yourself a long range round, which is difficult to control in automatic fire, doesn't permit as quick of follow up shots (assuming no muzzle brake), and you can carry fewer of them than a smaller caliber. Given that the VAST majority of rounds fired are misses (something like 25,000 rounds fired per casualties inflicted in WW2) or more probably suppressive fire, you're wasting the 'ideal round' not even hitting anyone to use it's wounding abilities. It is to keep their head down while you maneuver on them, call in fire support, or can use your squad/platoon intrinsic explosive weapons to kill, wound, or route them. At that point you're probably worse off than even using an intermediate 7.62 round. Jack of all trades is a master of none. As the US found out with it's combat experience there is simply no replacement for the 5.56, either smaller or larger, that is worth the cost of replacing it, because it fits it's niche so well, even with it's penetration/wounding issues (fixed now with the M855a1 round it seems). Range issues are something that would be solved by using a heavier caliber for select special weapons that can actually use it that have scopes or ability to fire a large volume of bullets with tracers to see where the bullets are landing at a distance. A ballistically better option already existing with the Swedish 6.5mm, which could be modified to a smaller case by using more modern powders, for direct fire while IMHO indirect fire was better done with weapons other than rifle caliber MG so a 'HMG' at the battalion level as in WW2 shouldn't exist....or if it did be something in a caliber like the 9x66mm MAS or the .338 Lapua used in the LWMMG...something that is able to penetrate against fixed positions or vehicles while being very long range without being as heavy or powerful as the .50 Cal M2.


Neither option really offers much, truth be told. The smaller calibers (6mm-6.5mm) will offer lighter cartridge weight (possibly also permitted a lighter rifle/weapon system) and increased control under automatic fire at the expense of "stopping power" similar to where NATO settled with the 5.54x45. It is because of this short coming that it may desirable or even required to keep a heavier .30+ caliber round for long range and heavy hitting requirements between that light infantry round and the 12.7x99mm (.50 BMG).

Regarding the .280, it also doesn't offer much in terms of universality that the 7.62x51mm doesn't also do, which is why I was advocating for a .270 caliber round which best balances the advantages of the smaller .22-.26 caliber rounds (light weight, low recoil, flat trajectories) and the .28-.32 caliber rounds (stopping power, momentum, resistance to drift/deflection, armor penetration at range, etc.). I guess, the summary is that if you are willing to accept a two caliber arsenal then take one from the lower end and one from the higher end--as NATO has had since the introduction of 5.54 to pair with the 7.62--where as if you are really serious about a "one-size fits all" universal round the best compromise is some in the 6.5mm-7mm range, with 6.8mm being the best balance of all. I was basing it on the premise of NATO not accepting 7.62x51mm as per the OP.
Even with something like say the 6.5mm round stopping power isn't going to be a big problem, but you will need a 'double version' standard, that is a short ranged round optimized for infantry hand weapons and one for MGs and sniper rifles like the German or Soviet intermediate and full power versions.
You'd probably be better off in terms of having LMGs/MMGs and sniper weapons, but with a not-so-optimal infantry hand weapon, plus lack of interchangeability between the long and shorter range rounds anyway. You'd really need a stubby and probably unbalanced 6.5mm bullet pushed as fast as possible to get that tremendous wounding effect the 5.56mm is known for (and apparently the 'Pig Board' discovered when they tried out the 6.5-7mm rounds for wounding ability on pigs; they also found that none had a super advantage over the others within 300m, but the 6.5mm round was the most damaging). They found the 6.5mm flat based bullet pushed to at least 800mps did the most damage, so I'd imagine if you got a 85 grain 6.5mm round to 850-900mps wounding power would be just fine. Recoil should be close to a .224 Valkyrie round (that is more powerful than the most powerful 5.56 NATO round, but significantly less than a 7.62x39 round), so you'd be good on that, but the kicker is not being able to carry many of them and having limited range performance due to designing the round for best wounding ability within 300m rather than aerodynamics for ranges beyond that.

All that said I don't get why the US in trying for 1000m performance optimization for the 7.62 NATO round didn't recognize the advantages of the 6.5mm round in that role other than desiring to minimize cost when adopting a new round by trying to keep the existing caliber and just modernize the cartridge.
What is even more inexplicable is the Brits doing the same with the .270-.280 rounds. They knew the 6.5mm round had the best ballistics and that the Swedes already basically perfected the long range bullets in that caliber if they were going to go for distance performance (the Swedish 140 grain 6.5mm bullet vastly outperformed the British 7mm/.280 140 grain bullet in distance performance) and given the US experience testing the wounding ability of the various calibers would have know that a 6.5mm round would wound better than a 7mm round within 300m (infantry combat distances), while retaining the best velocity out to long ranges if they wanted a 2000m MMG round (which they were trying to get the .280 round to do...without a boat tail for some reason).
 
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Thing is with the above, drag (in terminal ballistics) is not just function of speed. It also a function of the bullet itself. A bullet that's expanding, or tumbling will increases drag, a bullet that fragments and breaks apart* which each fragment itself splitting away from the initial trajectory will also have it's own drag coefficient. On top of that each of those things also increases tissue trauma in and of themselves without worrying about increasing drag.

A larger bullet will drag more as it move through the body as drag is also effected by the cross section of the travelling object as well as it's speed. A smaller bullet might shed is velocity proportional faster in response to drag, but a more massive bullet will still shed it's energy as well. So while mass might not directly be turned into transferred energy, a more massive bullet still increases the energy available for transferal by having more the energy available in the first place.

there's also the point that bullet mass and velocity are seldom completely separate values. (and that they also impact on other parts of the ballistic journey before terminal)!

Your basic point about getting bullet to to transfer all it's energy into the target before exiting is correct, it's just that's exactly what expanding bullets were designed to help with


*which of course is more lley to happen with fast small bullets so there lots of effects going on here in both directions.
Sure.
Terminal ballistics is complicated. The problem with military terminal ballistics, as opposed to hunting or police, is that military requirements tend to set a given penetration at given range as mandatory, and that makes designing bullets to expand or fragment more difficult. The reason why I like the idea of having diferent rounds for rifles and MG is that you can keep the penetration requirement high for the MG round, and lower it for the rifle, that can then use a round that is optimized for antipersonel use at under 300m and have less recoil.
If I can only have one round for both rifle and MG, I'd like it to be able to go through the doors of a light truck at a 1000m and still be lethal.
The current trend for widespread use of ballistic vests in all types of combat will impose rounds with good penetration.
 
what advantage does keeping .30-06/.303 or moving to something like .280 brit/7.92* 41 have over 7.62*51?
Neither option really offers much, truth be told. The smaller calibers (6mm-6.5mm) will offer lighter cartridge weight (possibly also permitted a lighter rifle/weapon system) and increased control under automatic fire at the expense of "stopping power" similar to where NATO settled with the 5.56x45. It is because of this short coming that it may desirable or even required to keep a heavier .30+ caliber round for long range and heavy hitting requirements between that light infantry round and the 12.7x99mm (.50 BMG).

Regarding the .280, it also doesn't offer much in terms of universality that the 7.62x51mm doesn't also offer, which is why I was advocating for a .270 caliber round which best balances the advantages of the smaller .22-.26 caliber rounds (light weight, low recoil, flat trajectories) and the .28-.32 caliber rounds (stopping power, momentum, resistance to drift/deflection, armor penetration at range, etc.). I guess, the summary is that if you are willing to accept a two caliber arsenal then take one from the lower end and one from the higher end--as NATO has had since the introduction of 5.56 to pair with the 7.62--whereas if you are really serious about a "one-size fits all" universal round the best compromise is something in the 6.5mm-7mm range, with 6.8mm being the best balance of all. I was basing it on the premise of NATO not accepting 7.62x51mm as described in the OP.
 

Deleted member 1487

Regarding the .280, it also doesn't offer much in terms of universality that the 7.62x51mm doesn't also offer, which is why I was advocating for a .270 caliber round which best balances the advantages of the smaller .22-.26 caliber rounds (light weight, low recoil, flat trajectories) and the .28-.32 caliber rounds (stopping power, momentum, resistance to drift/deflection, armor penetration at range, etc.). I guess, the summary is that if you are willing to accept a two caliber arsenal then take one from the lower end and one from the higher end--as NATO has had since the introduction of 5.56 to pair with the 7.62--whereas if you are really serious about a "one-size fits all" universal round the best compromise is something in the 6.5mm-7mm range, with 6.8mm being the best balance of all. I was basing it on the premise of NATO not accepting 7.62x51mm as described in the OP.
Is the 6.8mm round really best compromise? I know the British thought so, but compare the performance of the 6.5mm Grendel to the 6.8mm SPC. You can of course build bullet to tumble if you're concerned about wounding power, so the slightly wider caliber of .3mm isn't going to really off that much of an advantage in that regard unless you're just going to stick with a standard military FMJ design.
 
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I basically agree with Wiking's "6,5" post.
Sweden had a great caliber mix, with 6,5x55 for rifles and LMG (Improved BAR) and 8x66 for HMG. If they had selected 9x25mm for SMG it would have been a shootist dream army.
 
Swedish 6.5mm...
I personally love the 6.5mm and think it is just about perfect. A "shorter" 6.5mm as you describe would essentially be an earlier .260 Remington or 6.5 Creedmore. The reason I went with .270 / 6.8mm is because it has superior terminal ballistics to the 6.5mm with only a slight disadvantage in "in-flight" ballistics (and because the .270 British was an actual round that was proposed at the time unlike 6.5mm or 5.56mm). This gives you a round with at the common sub-300m combat range (include CQB) can stop a target with greater assurance than the OTL 5.56 or even the 6.5mm while still appeasing the US military brass who were of the opinion that infantry needed an 800m weapon. As I said, it is a compromise, and one that I think if they had pursued they would have been able to develop into a proper universal round suitable for light and medium duty, easing logistics, production, and supply, for all infantry use from the combat rifle, CQB Rifle, and LMG/SAW.
 

Deleted member 1487

I basically agree with Wiking's "6,5" post.
Sweden had a great caliber mix, with 6,5x55 for rifles and LMG (Improved BAR) and 8x66 for HMG. If they had selected 9x25mm for SMG it would have been a shootist dream army.
Well, I do think the 6.5x55mm round was too powerful for a rifle, just that the caliber could have been workable; IMHO the 6.5 Swedish was great as a long range sniper/MG round, but serious overkill for infantry basic rifles. An 'intermediate' version of it could have been workable as an assault rifle round, though less than ideal. IMHO the 9x25 was too powerful for an SMG. The 9x18mm was probably best given the point of an SMG.
 
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Britain did adopt the .280 and the No9 Rifle with Canada and Belgium to follow suit until Churchill put a stop to it. The French went with 7.5x54mm. West Germany wanted to go with the FAL from Belgium so probably would have gone with the Belgian .280 except that the Belgians would not sell them a licence. With Britain, Belgium and West Germany going .280 the Netherlands, Norway and Denmark are likely .280 users too in that ATL.

Whatever might have happened there was every chance that NATO might have had no formal standard small arms ammunition except through evolution.

There was an argument for going with the French 7.5x54mm as an existing round of known performance already in use.

One also has to be careful in speaking of the British .270/280 as if it was a single type. It moved from the intermediate .270 to get close and closer to 7.62x51mm to appease the Americans but the final iteration was formally adopted with the No9 Rifle to replace the No4 Rifle and MkV Sten.

Myself I would have just gone with the existing Italian 6.5x52mm with a better bullet. All the power one for which one actually has a use and allowing a lighter rifle, LMG or GPMG but the thread is not about personal fandom but national choices.
 
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compare the performance of the 6.5mm Grendel to the 6.8mm SPC
Yes, but I am not talking about 6.8 SPC with its short FMJ. I am talking about a .270 with a longer and heavier bullet, like those in the .270 Winchester, but in a shorter case of 46mm-51mm length. I may have had my weights mixed up as now I am looking and the long 6.85mm is more like 150gr, not 115gr. It will produce less recoil than the 7.62x51 while still holding most of the energy and momentum of the heavier round. I'm not dissing 6.5mm, because honestly, I love the caliber for hunting. I am just trying to find a "dream" round that would have fulfilling NATO's requirements, real world usability, ephemeral "stopping power", and the c.1950 contemporary US Military desire for long range power.
 

Deleted member 1487

Yes, but I am not talking about 6.8 SPC with its short FMJ. I am talking about a .270 with a longer and heavier bullet, like those in the .270 Winchester, but in a shorter case of 46mm-51mm length. I may have had my weights mixed up as now I am looking and the long 6.85mm is more like 150gr, not 115gr. It will produce less recoil than the 7.62x51 while still holding most of the energy and momentum of the heavier round. I'm not dissing 6.5mm, because honestly, I love the caliber for hunting. I am just trying to find a "dream" round that would have fulfilling NATO's requirements, real world usability, ephemeral "stopping power", and the c.1950 contemporary US Military desire for long range power.
The 6.8mm SPC uses a heavier bullet than the British .270
https://en.wikipedia.org/wiki/6.8mm_Remington_SPC
https://en.wikipedia.org/wiki/.270_British
The .270 bullet was 100 grains, which is pretty short or filled with a steel core. They could have used a heavier bullet, but it would be much lower energy and velocity than the equivalent 6.8 SPC round.
That said you're proposing a 6.8mm magnum round, probably in a 7.62 NATO case necked down, if you're asking for performance like the .270 Winchester:
https://en.wikipedia.org/wiki/.270_Winchester
150 gr (10 g) SP 3,000 ft/s (910 m/s) 2,998 ft⋅lbf (4,065 J)
If you're going with a 150 grain bullet with that level of muzzle velocity, it would be MORE powerful than the 7.62 NATO round, as the M80 bullet was 147 grains:
https://en.wikipedia.org/wiki/7.62×51mm_NATO
147 gr (10 g) M80 FMJ 2,733 ft/s (833 m/s) 2,437 ft⋅lbf (3,304 J)

The 6.8mm round would have to be much more aerodynamic and long to get that weight, so would have much superior range performance...but would be a pre-WW1 throwback in their efforts to get military magnum rifle rounds in 7mm:
https://en.wikipedia.org/wiki/.276_Enfield
165 gr (11 g) 2,800 ft/s (850 m/s) 2,872 ft⋅lbf (3,894 J)

Why not just go with the 6.5mm Swedish then? Much less recoil and general energy, while having better sectional density and therefore ballistic potential
https://en.wikipedia.org/wiki/6.5×55mm_Swedish
120 gr (8 g) BT 2,812 ft/s (857 m/s) 2,108 ft⋅lbf (2,858 J)
140 gr (9 g) SP 2,651 ft/s (808 m/s) 2,185 ft⋅lbf (2,962 J)
140.4 gr (9 g) DK 2,854 ft/s (870 m/s) 2,540 ft⋅lbf (3,440 J)
160 gr (10 g) EVO 2,559 ft/s (780 m/s) 2,266 ft⋅lbf (3,072 J)

Of course if we're going down that path there was the 6mm Optimal....
https://en.wikipedia.org/wiki/6mm_Optimum
Crist's specifications were that "6mm Optimum" ammunition achieve, with a 100 grain bullet:
  • velocity: 2,900fps (muzzle), 1149fps (1,200m)
  • energy: 1,867ft-lbs (muzzle), 293ft-lbs (1,200m)
  • flight-time to 1,200m: 2.21 seconds
  • deflection @ 1,200m in 10mph crosswind: 151 inches
  • maximum trajectory: 244 inches
...of which he noted, "...even with a conservative estimate for the muzzle velocity of the 6mm Optimum cartridge, computed data for 1200-meter velocity, flight-time, wind-deflection, and trajectory height are all greatly superior to both 5.56 and 7.62 NATO rounds."[1]
 
To be really different if there is no 7.62 X51 then the Spanish 7.92 x 40 CETME round might actually get a chance. it at least this round allows the Americans to say that they still have a rifle calibre round!
 
@wiking thanks for the links and backgrounds. It has been a while since I looked into all of these and apparently my memory is a little muddled on it and I obviously made some questionable assumptions based on that faulty memory.

In a perfect world, yeah, I'd like to see a short 6.5mm as the go-to infantry round. With all of this I was just trying to find something that would be best given the needs as understood at the time and 6.8 seemed the best compromise. Something that in a light military load with a 115gr-125gr bullet will offer the infantry all they could ever want for normal engagement but which can still be "built up" for use by snipers without needing a different cartridge all together. The reason why I stayed away from 6.5mm Swede (my personal favorite) is because it is too long to be seriously considered for modern Assault weapons and high RoF LMGs and the same is true of the .276 Pedersen. Really, you need to keep it shorter than the 71mm of the 7.62x51mm and ideally something more in the range of 60mm-65mm overall. Wiking is right on with the 6.5 Grendel, but again, I think in 1950 that may be too much of a stretch.
 
I personally love the 6.5mm and think it is just about perfect. A "shorter" 6.5mm as you describe would essentially be an earlier .260 Remington or 6.5 Creedmore. The reason I went with .270 / 6.8mm is because it has superior terminal ballistics to the 6.5mm with only a slight disadvantage in "in-flight" ballistics (and because the .270 British was an actual round that was proposed at the time unlike 6.5mm or 5.56mm). This gives you a round with at the common sub-300m combat range (include CQB) can stop a target with greater assurance than the OTL 5.56 or even the 6.5mm while still appeasing the US military brass who were of the opinion that infantry needed an 800m weapon. As I said, it is a compromise, and one that I think if they had pursued they would have been able to develop into a proper universal round suitable for light and medium duty, easing logistics, production, and supply, for all infantry use from the combat rifle, CQB Rifle,
@wiking thanks for the links and backgrounds. It has been a while since I looked into all of these and apparently my memory is a little muddled on it and I obviously made some questionable assumptions based on that faulty memory.

In a perfect world, yeah, I'd like to see a short 6.5mm as the go-to infantry round. With all of this I was just trying to find something that would be best given the needs as understood at the time and 6.8 seemed the best compromise. Something that in a light military load with a 115gr-125gr bullet will offer the infantry all they could ever want for normal engagement but which can still be "built up" for use by snipers without needing a different cartridge all together. The reason why I stayed away from 6.5mm Swede (my personal favorite) is because it is too long to be seriously considered for modern Assault weapons and high RoF LMGs and the same is true of the .276
The 7,62x45mm?
Comes from the wrong side of the iron curtain, but it would fit that bill nicely...
 

Deleted member 1487

@wiking thanks for the links and backgrounds. It has been a while since I looked into all of these and apparently my memory is a little muddled on it and I obviously made some questionable assumptions based on that faulty memory.

In a perfect world, yeah, I'd like to see a short 6.5mm as the go-to infantry round. With all of this I was just trying to find something that would be best given the needs as understood at the time and 6.8 seemed the best compromise. Something that in a light military load with a 115gr-125gr bullet will offer the infantry all they could ever want for normal engagement but which can still be "built up" for use by snipers without needing a different cartridge all together. The reason why I stayed away from 6.5mm Swede (my personal favorite) is because it is too long to be seriously considered for modern Assault weapons and high RoF LMGs and the same is true of the .276 Pedersen. Really, you need to keep it shorter than the 71mm of the 7.62x51mm and ideally something more in the range of 60mm-65mm overall. Wiking is right on with the 6.5 Grendel, but again, I think in 1950 that may be too much of a stretch.
The .276 Pedersen might have just been perfect for the attitudes of the day for all things, as it would have had less recoil than the .280 British and longer range due to bullet design:
http://wintersoldier2008.typepad.co...n-were-pipsqueak-cartridges-in-compariso.html
https://en.wikipedia.org/wiki/.280_British

With a muzzle brake and due to having a lighter bullet than the .280 round (more of the muzzle energy coming from the powder load, which could be harnessed by a muzzle brake), it would have potentially been light enough in recoil to be a true assault rifle.

Also why do you think high RoF MGs would be the way they'd go at that time? The trend in designs was lower RoF for LMGs in particular to make them more controllable in automatic fire, less wasting of ammo, and lower the rate of heat buildup. In fact the US trend in MGs was substantially lower than say the M16 (M60 RoF was 500-650rps while the BAR was even lower with an adjustable rate limiter to get real slow). Length of the round didn't really matter to RoF either, as the FG-42 using the 7.92x57mm had a RoF of 700-900rps.

(also I don't mean to rag on ya, just looking at all this in granular detail and using your posts as a jump off point)
 
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BigBlueBox

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If I can only have one round for both rifle and MG, I'd like it to be able to go through the doors of a light truck at a 1000m and still be lethal.
The current trend for widespread use of ballistic vests in all types of combat will impose rounds with good penetration.
I see zero reason why a standard rifle or squad machine gun should be expected to penetrate a truck at that distance. That's what shoulder-fired launchers, anti-material rifles, heavy machine guns, mortars, artillery, and close air support is for. After reading through the posts on this thread, I have to agree that 6.5mm or 6.8mm would have been the perfect sweet spot for a standard rifle round, and would have eliminated the need for both 5.56 and 7.62. If a round weaker than 6.5mm or 6.8mm is needed, that's what submachine guns and PDWs are for. And if a stronger round is necessary, you might as well go all the way and use .50 BMG.
 
To be really different if there is no 7.62 X51 then the Spanish 7.92 x 40 CETME round might actually get a chance. it at least this round allows the Americans to say that they still have a rifle calibre round!
As Spain does not join NATO until what after Franco dies, I doubt it would be an option
 
Sure.
Terminal ballistics is complicated. The problem with military terminal ballistics, as opposed to hunting or police, is that military requirements tend to set a given penetration at given range as mandatory, and that makes designing bullets to expand or fragment more difficult.

That's true (I was really replaying in response to the expanding vs. non expanding in general, but that itself changes when it comes to military rounds)

The reason why I like the idea of having diferent rounds for rifles and MG is that you can keep the penetration requirement high for the MG round, and lower it for the rifle, that can then use a round that is optimized for antipersonel use at under 300m and have less recoil.

I can certainly see the temptation to go with two rounds each tailored for the job each will do out of different weapons. But you also run into having to support two different rounds, and lose out on potential benefits like supporting a range of weapons in the same calibre. However as ever it's always trade off!.

If I can only have one round for both rifle and MG, I'd like it to be able to go through the doors of a light truck at a 1000m and still be lethal.
The current trend for widespread use of ballistic vests in all types of combat will impose rounds with good penetration.

Yep that's true too! (and will of course have an impact on your choice of weapons).
 
Odd thoughts:

Curious why folk advocate 6.8mm rounds as modern ideals and dismiss the .270/280 which are about the same size. Also curious how the older 6.5mm rounds are either characterised as too powerful (Swedish) or too weak (Italian/Japanese) when the differences in the cases are a length of 55mm down to 50mm all with older powders.

If you want power and a flat trajectory then the old 1910 age British .276 Enfield would work fine with 1950s powders and bullets. I would not choose it but it ticks the full power round boxes. I do note that when Britain looked a full power rimless round at the end of WW2 they went to the Mauser 7.92x57mm which they already made and used in the BESA.

The order of events is what do you want the bullet to do. Then what case etc. will make it do it and then what weapon will best fire it. In real life nations want it cheap. In Europe there is no NIH syndrome as long as you can manufacture it at home (excluding France) but there was, and is, a huge NIH syndrome in the USA.
 
Having professional soldiers that serve longer and get much better shooting training also. this allows them to make better use of the more powerful rounds. It also lead to better decisions regarding when to use full auto or aimed single shots.
Its interesting to note that, for example, Portuguese "comandos" use both 7,62mm and 5,56 rifles and LMG in Portugal, but when deployed to Africa or Afghanistan they choose to use their trusted old G3 rifles and MG3 GPMG, both in 7,62x51.
Regarding the logistic issue of having more than one type of round, once logistics became a "motorized sport" that's not as complicate as it used to be. The US army suplied both 30.06 ans .30 carbine, as well as .45 for SMG to Infantry units without problems.
For a german rifle unit depending on a horse driven supply cart in Russia it might be more critical to be able to swap round with the LMG team in the next fox hole.
 
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