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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As the amount of propellant increases in explosive capacity, the heavier the bolt needs to be to soak up the energy and have it move slow enough that it doesn’t shoot out the back of the weapon. At some point you’re going to need huge bolts and that’s damn near impossible to control.
That’s why the biggest caliber rounds seen on blowback firearms are pistol ones, 9mm Parabellum, 10mm/.40, and .45ACP.
Bigger than that, such as rifle caliber rounds, are too unwieldy and not safe for practical purposes. The solution which most firearms have utilized for over a century is delaying the energy of the explosion in the chamber and out the barrel until it’s just enough for the bolt to go back and forth, clearing the spent case and cycling the next round as it does. To delay the movement of the bolt there are many designs, from roller delayed on the HK MP5, to gas or piston driven like the AR-15 and HK 416 where some of the gas going out the barrel gets in a small channel that forces a mechanism to rotate the bolt out of a locked position. I recommend Forgotten Weapons YouTube channel to check out how those systems work. From a purely mechanical aspect it’s quite fascinating.
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