Recoil of bullet passing through firearm barrel

[dshield55]

I was on a firearms forum and a guy said something along the lines of this.

When you're handloading your own cartridges, if you use a mild powder load (less powder) in a handgun versus a hot load (more powder), you would think that shifts the point of impact of the bullet on the target lower since the bullet will be going slower. But in fact, the point of impact can increase a few inches at 20 yards because the slower bullet creats more muzzle rise from the recoil of the bullet as it passes through the barrel slower.
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Muzzle rise when shooting is caused by a moment of angle created from the difference of the point of contact between your hand and the gun and the bore axis not being in the same horizontal plane. The bore axis is always higher than your hand grip which makes the muzzle rotate upwards around the center of your hand grip.

Recoil in firearms is momentum.

I always assumed that there is zero recoil from a bullet until it and the powder gasses exit the barrel. My assumption is that there is no change in momentum in a closed system, and that momentum only changes when it exits the barrel making it an open system. The momentum of the bullet is definitely changing while it's accelerating in the barrel, but does that really create recoil before it exits the barrel or does the recoil happen after it leaves.

In a semi-automatic handgun, the barrel and slide remain locked and do not move until some point after the bullet has left the barrel.

 

[256bits]

You pull the trigger and the pin strikes the bullet casing causing the powder to ignite releasing gases. The gases exert a pressure on the bullet causing it to move through the chamber. At the same time the gases exert a pressure on the casing causing it to move backward. Since the casing is in contact with the gun, the gun will move backwards. Since your grip is below the barrel, a torque is induced around some point about which the gun rotates.

The imbalance of forces on the bullet and casing ( high pressure on one side, atmospheric pressure on the other, or your grip through the gun ) which results in movement.

 

[FactChecker]

While there is no NET change in momentum in a closed system, there are changes in the individual gun and bullet momentum, even in a closed system. The man's statement might be right, but not exactly for the reason you are asking about. The faster bullet will give more recoil than the slower bullet. But the slower bullet that stays in the gun longer might be more effected by the recoil, even if it makes less recoil. It takes time for the recoil to change the aim of the gun. It would take some study and experiments to see if he is right.

 

[dshield55]

To clarify the question: compared with a bullet shot with a hot load (more powder), is the more mild load of powder going to cause the gun to flip more than hot load before the bullet exits the barrel causing the point of impact of the bullet 20 yards down range to be higher?

The rationale being that if recoil is causing the gun to rise while the bullet is still traveling down the barrel, if it takes longer for the bullet to leave the barrel with a mild load, then the gun would have also rotated farther during that time too?

 

[FactChecker]

Yes, that is possible. I would be interested to see if he is right. I would trust an experimental result more than I would trust a theoretical calculation since there are a lot of things to consider.

 

[dshield55]

While there is no NET change in momentum in a closed system, there are changes in the individual gun and bullet momentum, even in a closed system. The man's statement might be right, but not exactly for the reason you are asking about. The faster bullet will give more recoil than the slower bullet. But the slower bullet that stays in the gun longer might be more effected by the recoil, even if it makes less recoil. It takes time for the recoil to change the aim of the gun. It would take some study and experiments to see if he is right.

Thanks, that was more what I was asking.

Another question I've asked on here before had to do with recoil over time. A faster bullet has more recoil because more momentum, but it's also going to take more hand strength to stabilize the gun since the recoil is spread out over less time. The slower bullet not only has less recoil because of less momentum, but that recoil is spread out over a longer time period taking less hand strength to stabilize. I wonder if that plays in this scenario as well.

 

[FactChecker]

Without more information, I would take the man's statement as true. It sounds like he has observed this in actual experiments and, because it is counter-intuitive, he probably saw convincing evidence before he would draw that conclusion.