The Greatest Ever Devised

Deleted member 1487

You might think so, but... google "M1E5 Garand". Instead of adopting yet another standard caliber that might look damned interesting to the boys in Ordnance.
Apparently it was a later war design:
http://www.luckygunner.com/lounge/m1-tanker-garand/

A 7mm M1 Carbine however would be very interesting. Chop the standard Pedersen round down to 40mm length with a 7mm standard bullet and combined it with the shorter barrel/folding stock and you'd have a winner IMHO. Now just add in an automatic feature and pistol grip and get an assault rifle...
 
Apparently it was a later war design:
http://www.luckygunner.com/lounge/m1-tanker-garand/

A 7mm M1 Carbine however would be very interesting. Chop the standard Pedersen round down to 40mm length with a 7mm standard bullet and combined it with the shorter barrel/folding stock and you'd have a winner IMHO. Now just add in an automatic feature and pistol grip and get an assault rifle...

I'd be worried about the taper and case capacity of a cut down Pedersen cartridge. This cartridge would start with about the same rim and base diameter as 7.62x39mm (.450" vs .447") but with significantly more taper (shoulder diameter .385" vs .396"). Box magazines for a cartridge like this would have an even more severe curve than AK magazines, which would make them even more difficult to carry. For comparison, 5.56x45mm NATO tapers from .377" at the base to .354" at the shoulder, while the 7.62x51mm NATO tapers from .470" at the base to .454" at the shoulder. These small tapers are what allow straight 20-round box magazines and only slightly curved 30-round magazines for both calibers.

Also, 7.62x39mm is already somewhat underpowered for the size of bullet it fires. Even 120 grain projectiles in carbine-length barrels (16") only reach about 750 m/s at the muzzle. 6.8mm Remington SPC, which fires typical 7mm (.277") bullets, also around 120 grains, has a similar problem, which is why it doesn't present much improvement over 5.56x45mm NATO at long ranges. Light bullets at these calibers can gain velocity but will lose sectional density, so there is definitely a tradeoff.

Overall, the smaller cartridge is a good idea for a very light backup carbine like this, but this is probably oversized for that purpose while being too small to serve as a standard infantry cartridge. The real killer, however, would be the severe case taper, which would make magazines quite large, heavily curved, and difficult to handle.
 
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Deleted member 1487

I'd be worried about the taper and case capacity of a cut down Pedersen cartridge. This cartridge would start with about the same rim and base diameter as 7.62x39mm (.450" vs .447") but with significantly more taper (shoulder diameter .385" vs .396"). Box magazines for a cartridge like this would have an even more severe curve than AK magazines, which would make them even more difficult to carry. For comparison, 5.56x45mm NATO tapers from .377" at the base to .354" at the shoulder, while the 7.62x51mm NATO tapers from .470" at the base to .454" at the shoulder. These small tapers are what allow straight 20-round box magazines and only slightly curved 30-round magazines for both calibers.

Also, 7.62x39mm is already somewhat underpowered for the size of bullet it fires. Even 120 grain projectiles in carbine-length barrels (16") only reach about 750 m/s at the muzzle. 6.8mm Remington SPC, which fires typical 7mm (.277") bullets, also around 120 grains, has a similar problem, which is why it doesn't present much improvement over 5.56x45mm NATO at long ranges. Light bullets at these calibers can gain velocity but will lose sectional density, so there is definitely a tradeoff.

Overall, the smaller cartridge is a good idea for a very light backup carbine like this, but this is probably oversized for that purpose while being too small to serve as a standard infantry cartridge. The real killer, however, would be the severe case taper, which would make magazines quite large, heavily curved, and difficult to handle.
I suppose then there is the historical option: adopt the historical carbine and cartridge, but neck it down to 7mm instead of 7.62. Johnson was able to get a 5.7mm projectile to work in it:
 
Apparently it was a later war design

Yes it was. Garand knew better, so he never really tried to develop anything like it, but was more or less cajoled into trying later in the war. It failed, as he knew it would, because the .30-06 is so powerful. But with the PD42 .276 round he might be tempted to try something like it a bit earlier- and it probably would work. Certainly, Ordnance might be willing to not specify "based on the .32 rimless self-loading cartridge" in their request for carbines to evaluate, to see if a Garand folding-stocked carbine might fit the bill. And if they did so, I bet Garand would bash something like the M1E5 together for the trials.

A 7mm M1 Carbine however would be very interesting. Chop the standard Pedersen round down to 40mm length with a 7mm standard bullet and combined it with the shorter barrel/folding stock and you'd have a winner IMHO. Now just add in an automatic feature and pistol grip and get an assault rifle...

I could make up all sorts of de novo stuff that might make the M1 Carbine more interesting, but I'm trying to minimize just making crap up. For the same reason the 6.5mm Mannlicher would be my choice for a *FG42, unless I could find some VERY good data about those experimental Luftwaffe 7mm rounds you mentioned.

Where I'm going to run into trouble is Vietnam-era developments. Because there was so much weird shit floating around and being tried, and it's hard not to unleash the butterflies in that environment.

6.8mm Remington SPC, which fires typical 7mm (.277") bullets, also around 120 grains, has a similar problem, which is why it doesn't present much improvement over 5.56x45mm NATO at long ranges. Light bullets at these calibers can gain velocity but will lose sectional density, so there is definitely a tradeoff.

Yeah, I'm critical of 6.8mmSPC, too. But the fanbois (read: 5.56mm-hater mafia) can be hard to convince. 6.5mm Grendel is much more interesting. But it's pretty much impossible to get anything truly interesting to fit through the magazine well of an M16.

I suppose then there is the historical option: adopt the historical carbine and cartridge, but neck it down to 7mm instead of 7.62. Johnson was able to get a 5.7mm projectile to work in it:

Yeah, we had a really long discussion about .22 Spitfire on another forum post here. It probably would be more interesting than .30 Carbine, but what it most definitely would not be is a *5.56mm. It had significantly less case volume and would never be able to generate the muzzle velocities needed to replicate the tumble-and-fragment mechanism that made the 55-grain 5.56mm so deadly. It would be much more like a *5.7x28mm than a *5.56mm.

So, it just pokes holes in things. Like the .30 Carbine, or a handgun. So, more interesting yes, but not really a great improvement.
 
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Deleted member 1487

I could make up all sorts of de novo stuff that might make the M1 Carbine more interesting, but I'm trying to minimize just making crap up. For the same reason the 6.5mm Mannlicher would be my choice for a *FG42, unless I could find some VERY good data about those experimental Luftwaffe 7mm rounds you mentioned.
I've got a British weapons magazine with the full experimental data from a translated German report, so ask away. Also I have another book about that talks about the variety of intermediate caliber bullet designs prior to the 7.92 Kurz, including a cartridge that was 45mm long with either a 7mm or 7.92mm bullet, but unfortunately not experimental data like I got for the 7mm Luftwaffe rounds. I can estimate performance though and the 7.92x45mm round would probably be an ideal compromise from a production stand point and range/power standpoint assuming a mild steel cored bullet like the 7.92 Kurz (but longer with better form factor).

Yeah, we had a really long discussion about .22 Spitfire on another forum post here. It probably would be more interesting than .30 Carbine, but what it most definitely would not be is a *5.56mm. It had significantly less case volume and would never be able to generate the muzzle velocities needed to replicate the tumble-and-fragment mechanism that made the 55-grain 5.56mm so deadly. It would be much more like a *5.7x28mm than a *5.56mm.
I'm not advocating a 5.7mm Spitfire for a carbine here (though IOTL Johnson was able to get that fragmenting effect with a super light 5.7mm round, lighter than a 5.56mm 55 grain actually, and IOTL the very first 5.56mm SCHV experiment was conducted with a M2 Carbine in the 1950s, the Gustafson round), just showing that you could neck down a round aggressively without having feeding issues.
A Pedersen 'Kurz' for a carbine version shouldn't be a problem if the Germans could get theirs to work with a MUCH greater reduction in length for their round.
 
Presuming the carbine variant is developed, it seems likely a new round will be desired (if not essential) to counter the excessive blast of the standard hardball .276, something with a faster-burning powder (which suggests a shorter round might as well be developed, too). The issue then becomes, is the bullet weight kept relatively constant? If it is, there might be excessive chamber pressures, leading to everything from battering due to recoil right up to receiver cracking. (I would consider the latter improbable, unless Ordnance wants very high MVs & MEs, not an impossibility...)
 
Presuming the carbine variant is developed, it seems likely a new round will be desired (if not essential) to counter the excessive blast of the standard hardball .276, something with a faster-burning powder (which suggests a shorter round might as well be developed, too). The issue then becomes, is the bullet weight kept relatively constant? If it is, there might be excessive chamber pressures, leading to everything from battering due to recoil right up to receiver cracking. (I would consider the latter improbable, unless Ordnance wants very high MVs & MEs, not an impossibility...)

Well, the biggest complaint with the .30-06 M1E5 was indeed the excessive muzzle blast. But the .276 might be a little less disagreeable- it's significantly less powerful than .30-06. Modern hobbyists love bashing together "tanker Garands" that don't have a barrel much longer than the M1E5 and they seem to like them just fine, so a slightly shorter .276 Garand would probably be tenable. I have plans for at least an attempt at a down-powered cartridge, but I don't know if I'll have it adopted or not. I'm still thinking about it. Or I might just mount an M3 Grease Gun-style flash suppressor or something.

FWIW the M1E5 does have two advantages over OTL M1 Carbine: it uses an already-standard and more powerful cartridge, and when folded it is damned compact. I think it would be competitive in the trials.

Also, another interesting POD- for someone else to explore- is the Winchester G30, G30M, and M2 rifles. Very interesting. The M2 was a 7.5-pound .30-06 prototype! (It was also- and this is a really interesting story if you want to look it up- the predecessor to OTL's M1 Carbine.) So, maybe "Carbine" Williams redesigns the G30 into the M2 earlier, in time for the USMC trials, and it is adopted instead of the Garand? Especially if it is in .276 rather than .30-06. Hmm...
 
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But the .276 might be a little less disagreeable- it's significantly less powerful than .30-06. Modern hobbyists love bashing together "tanker Garands" that don't have a barrel much longer than the M1E5 and they seem to like them just fine
AIUI, the issue isn't power, it's powder: the Ordnance ball rounds using a slow(er)-burning powder than the shorter-barreled carbine likes. Modern shooters can select ammo designed for the shorter barrel, or load their own, using a faster-burning powder, & so avoid the issue. All I'm saying is, the Army might end up with a *.276 Carbine round, with a (slightly) faster-burning powder & (slightly) lighter slug, to avoid blast & chamber overpressure issues.

That this might lead to a postwar equivalent of the 7.92mmK...
 

Deleted member 1487

I could make up all sorts of de novo stuff that might make the M1 Carbine more interesting, but I'm trying to minimize just making crap up.
Butterflies result from PODs, which mean some stuff can be invented that didn't exist IOTL.

For the same reason the 6.5mm Mannlicher would be my choice for a *FG42, unless I could find some VERY good data about those experimental Luftwaffe 7mm rounds you mentioned.
To avoid the mess of the caliber change, you could have the Luftwaffe also opt to go for the 7.92x45mm round Polte developed by 1941; the case capacity was almost exactly the same as the Italian 6.5mm Carcano or the 6.5 Arisaka. With a 9 gram version of the bullet with a steel core, it could well have chemical performance similar to the Mannlicher case, but of course the bullet would be different; supposedly the Germans tried mounting a 7mm bullet on the same case necked down according to some pictures I've seen and they had developed 9mm high BC bullet for it.

It's not really what you're looking for, given the Germans were unwilling to caliber shift in the middle of a war a 9g 7.92mm bullet on a 45mm long case is probably the way they'd go, but it isn't quite as hard a conversion as the regular German rifle round. Perhaps Johnson's LMG might get more interest post-war as a GPMG mechanism starting point, especially if already in 7mm Pedersen?
Not that much different than the FG-42.

Edit:
It also was adapted to a belt-feed mechanism:
http://www.forgottenweapons.com/belt-fed-johnson-lmgs/

Where I'm going to run into trouble is Vietnam-era developments. Because there was so much weird shit floating around and being tried, and it's hard not to unleash the butterflies in that environment.
Unleash them.
 
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What about using just the cavalry carbine? The rifle is handy, but the carbine would be handier still. 21-22 inches is the acceptable minimum barrel length for non-magnum .284-caliber rifles, right? At the very least, the rifle could be issued to snipers while everyone else gets the carbine.
1793.jpg
 
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