Gut check possibly dumb recoil question.

[Macintosh]

Talking calculated recoil here. It involves the mass of the gun, the mass of the ejecta (bullet+powder) and the velocity.

My question is related to the effect of barrel length on velocity, with ALL other factors staying the same.

If I took the same gun, same lot of ammo, and shortened the barrel, it loses about 25fps per inch.

If the gun weight magically stayed the same and shortening the barrel 8” was the ONLY change made, the recoil energy goes DOWN because the muzzle velocity is lower. Same gun, same cartridge, just one has a 16” barrel and the other has 24” barrel. The 16” gun will recoil less. (We are disregarding muzzle blast, etc).

Correct? If not, what am I missing?

 

[crazygrizz]

Maybe a smidge. But for the most part once the blast is directed your getting a force pushing the opposite way.

 

[alexnelon]

Kind of but you can't really feel it. It's like 0.5-1% tops which is more than compensated for by the weight of the barrel.

We're talking about a difference that is proportionate to the roughly 0.00008 second difference in the time it takes the projectile to leave the 16 inch barrel vs the 24.

 

[RockAndSage]

Those calculations will have been worked out based on a relatively full burn of the powder charge.

The problem is, the shorter the barrel, the higher the gas pressures at the muzzle.

Also, when the expanding gasses and fuels exit the muzzle of a gun, they get a sudden kick of oxygen from the open air - gas temps go from about 1500-2000F at the end of the bore, to about 2500-3000F 6 inches out past the muzzle. This is enhanced even more in an SBR. The more unburned fuel there is right there, with all that new oxygen, you also just get a different muzzle blast. It's more concussive, and to some degree, more violent and more instant. Some of that will be happening while the bullet is still part of the equation between forces pushing against it and against the muzzle.

I'm not sure how much that translates to recoil, but something is definitely happening there that's different from a more complete burn out of a longer barrel.

Another element that definitely impacts recoil, is surface-area right at the muzzle. Old style donut-shaped muzzle crowns wrapping around the bore and the barrel exterior are some of the better in minimizing the force of the gasses against the barrel's muzzle as they escape under high pressure, still pushing against the bullet right as it exits. A flat-faced bull barrel muzzle experiences much more "recoil" force because of that vertical surface area, but the weight of that bigger barrel will offset felt recoil a bit. The absolute worse type of muzzle crown for recoil is the utterly moronic rocket-nozzle crowns put on some big-bore revolvers - it enhances recoil the same exact way a rocket nozzle's shape is used to enhance thrust. Interestingly, that type of crown can also enhance reliability on some semi-auto handguns, depending on how they're sprung and if they're ported, by increasing backward thrust on the moving parts to keep them at an appropriate cycling speed.

Bottom line on SBRs though, is you get much higher gas pressure at the muzzle, and a much more violent air-fuel burst as those fuels and gasses reach open air, both of which are altering recoil differently than a longer, more complete burn, in a longer barrel.

 

[Castle Rock]

Muzzle blast has a worse influence on your shooting than recoil

 

[Macintosh]

OK, I guess that's my question. For the moment lets assume we add weight to the gun to account for the reduced weight of the barrel, so gun weight is constant.

here is +/- a 6.5cm from a 24" vs a 16" (2700fps vs 2500fps). In this look it's an 8.8% reduction in recoil energy. Is there actually more to it than this?

re: muzzle blast and concussion, lets assume both barrels are suppressed.

 

[crazygrizz]

Seems like you've answered your question. Not really anything more to it since we're are taking away all the real life variables

 

[Marbles]

Yes.

Due to equal and opposite reaction forces (Newtow's Third Law), just as a slower moving projectile carries less energy, less energy will be imparted to the rift system.

My 17 inch 243
2.28 lbs.sec recoil impulse
9.15 fps recoil velocity
10.41 ft.lb recoil energy

If I gained 200 fps with a longer barrel, but keep the weight the same.

2.37 lbs.sec recoil impulse
9.54 fps recoil velocity
11.3 ft.lb recoil energy

So, an 8.5% increase in recoil energy from adding about 7 inches of barrel.

Now, and other things like muzzle blast could all come together to make the perception of recoil be higher with a shorter barrel.

If we figure 7 inches of barrel equals 8 ounces, then:

2.37 lbs.sec recoil impulse
8.97 fps recoil velocity
10.64 ft.lb recoil energy

The weight almost certainly offsets any perception of the slight increase in energy due to lower recoil velocity.

Edited to correct a brain fart on impulse.

 

[Macintosh]

Seems like you've answered your question. Not really anything more to it since we're are taking away all the real life variables

yes, understood--my 18" unsuppressed 30-06 has me extremely aware of those factors! I'm just eliminating the variables to get a more specific answer. In the planning stages of building a gun and for some reason got hung up on this in my own head and just looking to clarify my understanding is all. I appreciate you responding.

If I do the math accounting for barrel weight it's still a very slight recoil savings on paper, but not enough to differentiate for sure. 0.36 ftlb in the example calculator. And this will be suppressed when possible, but even when not, that fire-breathing dragon I mentioned^^ is the benchmark, so this should be mild by comparison. Again, just confirming my understanding of the topic on paper.