why automatic rifles can’t use blowback reloading systems

Question

I’m not super familiar with firearms and usually only see them in video games. I notice that pistols and sub machine guns use something called blowback to load the next round from the magazine. My understanding is that the recoil of the round pushes the bolt back and ejects the bullet. This works for pistols and submachine guns like a Glock or MP40.

For rifles though, they have to be gas operated or put in a separate tube so that the gas can go backwards to push the bolt back. Why is this the case? Why can’t rifles like the M16 or AK47 simply use blowback and let the recoil of the gun push the bolt back and load the new round?

In: 26

Answer 1

They do have rifles that use these, Spanish Cetme and it’s upgraded clone the HK G3 series do this. I am no expert and anybody who is that sees anything wrong with my explanation feel free to correct me. Rifles calibers have a lot more chamber pressure than pistol calibers which attributes to the projectile leaving the muzzle at higher speeds. This means your bolt has significantly more pressure on it when firing which means it will move back at a higher speed unless you bleed off some of that pressure first. barrels are intended to funnel all the gas out the muzzle end to impart as much force on the bullet as possible to raise the speed of the bullet. If the bolt opens prior to the bullet leaving the barrel gas can now go the opposite direction and your bullets velocity will be impacted. Guns like the HK G3 use rollers which are designed to delay the bolt from moving backwards until after the bullet has left the barrel which allows for some of the pressure to be released prior to the bolt unlocking which allows for less force on the bolt and the most efficient velocity for the bullet. Rifles with gas or piston systems funnel gas further down the barrel allowing for the bullet to leave the muzzle by the time the gas/piston causes the bolt to release. Ultimately it seems that piston or direct impingement systems are easier to get dialed in than delaying the bolt moving through mechanical parts.