GrumpyOlGuy
Member
- Joined
- Aug 26, 2018
- Messages
- 15
More on the conversion of an M1 Carbine to a "Short Blackout:" original post from 2018. The bolt face was opened up by an additional 0.035-inch diam., to accommodate the slightly larger cartridge base. The mods to the bolt and chamber don't appear to be a significant margin impact/risk (still adequate metal remaining). The case is about 1.20 inch in length (slightly shorter than the M1 Carbine case = 1.28 inch). The mods were to current manufacture hardware, mated to a modern forged/machined receiver.
As to pressure signs: Nothing noticeable in primer flattening or firing-pin dimple up to about 13.0 grains of WC820 (same powder gives USGI spec 1970 fps in a standard M1 Carbine). However, the velocity achieved (around 1930 fps) was lower than expected. Not sure why this is. Need to gain insight if the pressure would increase with the slight cartridge bottleneck.
However, because of the bullet's higher BC (0.293 (claimed) compared to the standard 0.178), the bullet still achieves 9mm Parabellum MV and ME at about 350-375 yards. This compares to an effective range for the standard M1 Carbine of about 200-250 yards.
The big downside for this round is still the bullet drop at longer ranges: 20 inches at 250 yds; 36 inches at 300 yards; and 55 inches at 350 yds. That, along with the only moderate accuracy of ~2.5 MoA, would be the main limiters for the use of this round. The rounds fit and feed from a standard magazine.
So - in the end, a slight increase in the rifle's design (cartridge length to about 2") to permit a higher BC bullet would have made the little rifle a bit more powerful at range. It's not bad bang for the weight (a little more than 5 lbs). Still not a battle-rifle, but more of a true assault rifle.
As to pressure signs: Nothing noticeable in primer flattening or firing-pin dimple up to about 13.0 grains of WC820 (same powder gives USGI spec 1970 fps in a standard M1 Carbine). However, the velocity achieved (around 1930 fps) was lower than expected. Not sure why this is. Need to gain insight if the pressure would increase with the slight cartridge bottleneck.
However, because of the bullet's higher BC (0.293 (claimed) compared to the standard 0.178), the bullet still achieves 9mm Parabellum MV and ME at about 350-375 yards. This compares to an effective range for the standard M1 Carbine of about 200-250 yards.
The big downside for this round is still the bullet drop at longer ranges: 20 inches at 250 yds; 36 inches at 300 yards; and 55 inches at 350 yds. That, along with the only moderate accuracy of ~2.5 MoA, would be the main limiters for the use of this round. The rounds fit and feed from a standard magazine.
So - in the end, a slight increase in the rifle's design (cartridge length to about 2") to permit a higher BC bullet would have made the little rifle a bit more powerful at range. It's not bad bang for the weight (a little more than 5 lbs). Still not a battle-rifle, but more of a true assault rifle.
