Recently I built an AR based 9mm PCC. While going down the rabbit hole of buffer configurations (thanks blowback9.com), I read the relationship of lighter recoil with shorter barrels. I am referring to blowback operated, so gas system length, RDB, etc do not apply.

Now, I’m not an engineer, but I did stay at some motel once before, and what I know about the laws of physics is basically everything action has an equal and opposite reaction, blah, blah blah.

So given a carbine weighing X, a recoil system with a given mass on a spring with a given rate, a set amount of travel, the felt recoil will be the remainder of the energy produced from the expulsion of the projectile minus that dissipated by the recoil system over time. F=ma, right?

Upon ignition the bullet is expelled forward under pressure; the case rearward, delayed by the mass. The bullet’s mass and explosive energy of the propellant charge is the opposite end of the equation. Is the time  spent by the projectile in the barrel really part of the equation for an appreciable difference in felt recoil?



I suppose as long as the bullet is accelerating it would be. The time factor of the bullet under acceleration is the opposite of using more mass and stiffer springs (or hydraulic dampers) to disperse energy over time for less felt recoil.

Just thinking out loud. Thanks!

My thought? Not science by any stretch but..

Longer barrel, longer time pressure is trapped, faster bolt speed increasing felt recoil? Versus short barrel, letting off pressure much faster.. energy wasted in muzzle flash

That was pretty much the epiphany I had while writing the post… epiphany.gif failed to load

My take as well ^^^

A more complete treatment of the subject can be found at the link below.  In particular, in the text at column 3, line 65 through column 5, line 10.
https://ppubs.uspto.gov/dirsearch-public/print/downloadPdf/9146069

MHO, YMMV, etc.  Be well.

Good info, thanks for posting that!

You are very welcome.

Mine as well


Would be interesting for someone to do a test if a couple muzzle devices at 16" and bare
Then cut back 1" until bullet sticks out of barrel
On SBR ofcourse

Model after the muzzle break tests to see how much it pushes the rig back with each length