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?
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The principle is called Recoil operation. In pistol calibre weapons this can be simple blowback, but for larger rounds some form of delay mechanism is required.
The issue is Chamber pressure, when a bullet is fired Burning gasses expand from the bullet case and push the Projectile up the barrel, Recoil does not begin until the projectile leaves the barrel, in an ideal system the pressure from the expanding gasses should reach its maximum just before the projectile leaves the weapon,
A 9 X 19 mm round has 560J of energy, so the Bolt and spring only have to be heavy enough to keep that pressure back until the recoil happens.
.308 rifle rounds have 3600J of energy this would require a very heavy bolt and spring, so a method is required to make sure the breach (that’s the bit where the explosion starts) remains closed until the bullet leaves the barrel and pressure drops, some of the methods for keeping the breach locked are Toggles, Rollers, Flaps, Tilting block.
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