300 Blackout,asa Whisper, has 24 grains water capacity vs 20 of the carbine.
Some describe the 300 BLK as a way to get 45ACP energy delivered accurately at 150 yards.
Perhaps. Thing is the muzzle velocity is going to be very low due to how much space the bullet will take up in the base, while the weight of the bullet will increase the recoil in such a light platform even with the low muzzle velocity...150 yards accuracy is probably optimistic with that case and bullet.
30 Carbine
Overall Cartridge Length (IN) - 1.7
Case Length (IN) - 1.28
Full Case Capacity (GR WATER) - 20
Bullet Length (IN) - .72
Bullet Seating Depth (IN) - .3
Displaced Water (GR) - 5.641
Effective Water Capacity Of Case (GR) - 14.359
Any deeper seating, and you are looking at compressed powder loads. That's not good. So you are looking at pistol powders, need to find one that likes no void space in the case.
Easier is to toss the OAL limit, and go longer, but the limited seating area rules out most boat-tail designs
It doesn't kill you to have multiple bullet designs, especially since why would you bother to boattail a tracer? need all the space you can get to keep weight somewhat close to your ball and ap loads
They did have the 150 grain bullet high pressure test load, but a test cartridge to proof the barrel is quite different from a mass use service round. I'd imaging using the ball powder isn't an issue for the .30 Carbine cartridge with the M1 bullet, but that would mean a seriously reduced powder load to maintain the relative pressure. Of course at that point there really isn't a benefit to the 18 inch barrel and anything over say 10-12 inches could be a detriment due to the fall in pressure at a certain point once the powder is burned up and the needless friction resulting from bullet-barrel contact. That's why subsonic bullet platforms have short barrels.
If you're going to toss the OAL, might as well make it bottlenecked cartridge like the German 8mm Kurz. Just use the .30-06 case and chop it down to say 36-39mm to keep the case taper within reasonable limits. Of course then you've gone beyond what the Carbine platform could handle at that weight and you're back to having a system in the 3.5 kg range, as you've basically created an equivalent to the 7.62x39 cartridge, but more powerful and difficult to control in automatic due to the weight of the bullet unless you install a good muzzle brake (not FG42 good, but better than the M14 one). One the plus size you could basically use a scaled down Garand gas system to make the design task easier...
Or you could just use the rimless .351 WSL case and the M2 Ball bullet with a mild steel core to get it down to 125 grains and scale up the M1/2 Carbine to 6-7 lbs to deal with the heavier cartridge.
Regardless the M1 Ball bullet and .30 Carbine case aren't going to work well beyond creating a subsonic round for close range ambushes/sniping.