Originally posted by Vukovlad
Why would being a blowback be a problem for a military rifle?
Straight blowback firearms are generally used for pistols and submachine guns, where accuracy isn't as much an issue; when the round fires, the reaction pushes the bolt backwards and the casing goes with it - but this is happening at the exact same time the bullet is still in the bore going out the other end of the gun. As the casing ejects, you lose the seal inside the chamber/barrel and thus rob the bullet of velocity.
This is why most successful military semiautos and autos have that long gas rod that connects all the way out at the end of the barrel. The bolt stays closed and the casing in the chamber until the bullet has reached the muzzle and the gas pushes back the rod - only then does the bolt go back and eject the empty casing. Higher muzzle velocity, more accuracy. A reason bolt-action rifles have always been and always will be more accurate than semi-autos: there is no blowback whatsoever, and no moving parts, and every bit of the gas pressure keeps pushing on that bullet until it's out the barrel.