Alternate Weapon Development

what is the smallest you could conceivably get a bullet before it becomes ineffective or too difficult to manufacture the rifled barrel?
Well, saboted 1.5mm Flechettes were tested at over 4700fps for the ACR program in the late '80s
Since it's around 10 grains in weight, it's not much of a bullet
 
A couple of very esoteric points - when you go much below 5mm you apparently run into an issue where water that gets into the bore won’t reliably drain out unless you open the chamber end. So when you crawl through a puddle, fall in a stream etc you need to tip the weapon and cycle the action and be quite deliberate in draining it or else risk a malfunction when you pull the trigger.
Regarding propellant, the grains/flakes are quite carefully sized/shaped since they burn at the surface and this regulates the speed. If you basically have a great big pellet it may burn too slowly or explode instead of burning. It’s certainly possible to do such things, the black powder load of .303 apparently had one big lump of compressed propellant, but fiddling with the combustion surfaces of a given propellant may have very strange effects on the pressure curve behind the bullet and unexpected results ranging from the weapon exploding to a sad fart which leaves the bullet stuck in the barrel.
 
@RamscoopRaider, good point. Perhaps we can go a different route with the tanks and have one that is like a more conventional tank but with a bigger engine. So a medium battle tank that is faster and a bit larger as a result. Got a decent punch but the extra speed allows the tanks to group together in larger groups much faster for a defeat in detail or allows each tank to more effectively intercept an enemy attack. The extra speed would also allow formations to choose their battles, such as retreating when they are losing and having their enemy not be able to catch up. The speed means that enemies will find it hard to retreat. Same conventional medium tank gun and armour, extra cost and bigger profile (perhaps looking like a heavy tank due to the large engine size) but very fast.

About the 4mm bullet, thanks for that.

On another note when you look at a bullet the gunpowder granules seem to have a bit of air space between them. Would it be worth it to pour some gunpowder like substance into the cartridge so it perfectly fills the cartridge with no air spaces? The point is to potentially increase the power of a piece of ammunition without changing the dimensions at all by removing all the air spaces. You could also try having ammunition with the same power but is now able to be smaller due it having no airspaces inside the cartridge. You could use the stuff they use on caseless ammunition which seems to be able to be moulded into shapes.
Okay then you tank formations get immobilized by air attack because you put your fighter engines in tanks and lost the air war. Tanks generally got among the best available engines that weren't needed more vitally elsewhere in OTL. One supposes you could put two engines in one tank, but then you need a more complicated and stronger drivetrain and have more things that can go wrong and lead to worse reliability, and may be the bottleneck in manufacture. Tanks generally did not drive long distance but were moved by rail or transporters, because they tend to break down pretty often as is

There is a reason for the airspaces in gunpowder granules, to control burning time. You need more advanced propellants to do without that, and those have not yet performed as promised
 
One supposes you could put two engines in one tank, but then you need a more complicated and stronger drivetrain and have more things that can go wrong and lead to worse reliability,
...
Tanks generally did not drive long distance but were moved by rail or transporters, because they tend to break down pretty often as is
The Chrysler Multibank of 5 six cylinder engines all together was surprisingly reliable, far more than the Radial.

The GMC Twin Diesel in M4s and M10, plus the gas Cadillac twins in the M5 Light were all very reliable, as were some of the Soviet lights that used twins.

US tanks with the rubber block 'live' track were about the only ones in WWII that didn't need transporters, the Sherman Track had a long rated life than the Engine in a T-34
 
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