The differences between the military ammo and commercial ammo is in two areas, headspace and capacity. The external dimensions will be nearly identical, but the military cartridge will have thicker walls, especially at the base of the cartridge which will lead to higher pressures due to the slightly smaller capacity, assuming the same powder charge. The headspace is usually different as well. No, I don't know why, it's just the way that the military works I guess. These differences are the same with 7.62 X 51 Vs .308 Win., 5.56 Nato Vs. .223 and even the venerable .30 Govt. Vs 30-06. The simple solution is to download the military brass a small amount to make it shoot the same as the commercial brass cartridges. How much? For 7.62 Nato I reduce the charge by 1.4 grains. I don't reload pipsqueek bullets, so I don't know, you'll have to experiment a little.