In the comments to A Revolver Shooter’s Lament a new reader to Gun Nuts asked why heavier bullets would hit higher than lighter bullets. My initial response was snarky and sarcastic, but upon some thought I realized that I could provide a proper answer, and so I edited my comment to do just that.
To understand what causes the revolver to act the way it does, you have to remember that in a revolver, none of the recoil energy of a cartridge going off is being used to operate the gun. Thus, when the cartridge goes bang, the effect of recoil is immediately applied to the shooter, causing the gun to begin to pivot in recoil while the round is still in the barrel. While this only last for fractions of a second, those fractions of a second are enough time to change where the round will impact. Let’s look at two bullets of different weight and different muzzle velocities as an example.
A 158 grain bullet fired at 750 FPS is traveling at 9000 inches per second. At that speed, it will take approximately 0.000333 (repeating) for the bullet to travel three inches of barrel. This disregards chamber gap, forcing cone, etc. A 125 grain bullet at 900 FPS (10800 inches per second) will travel the same distance in 0.0002778 seconds. Finally, a 110 grain bullet at 1100 FPS (13,200 inches per second) will travel 3 inches in 0.00022728 seconds. Again, we’re dealing with tiny fractions of seconds here, but they do make a difference.
The physics become more complicated when you’re trying to figure out how much exactly the difference in velocity will affect the point of impact, but a good rule of thumb is the lighter the gun, the bigger the difference. Also, having weight forward of the revolver’s pivot point will also help mitigate this issue. For example, when I’m shooting a 2 inch j-frame, the difference between a 110 grain JHP at 1100 FPS at a 158 grain LRN at 730 FPS is profound, out of a six inch Security Six it’s not as bad.
Writing this post did make me wish I had a bunch of money so I could go buy a proper high-speed camera and film this at 10,000 frames per second and really show you the difference. Clearly, I should start a paypal fund and accept donations to the Buy Gun Nuts A Phantom camera!
Get in touch with Jason at Smarter Everyday on YouTube. He has done some amazing high speed photography.
If you try to laser bore sight a revolver you will find it much lower than an auto. Evidently recoil has much more effect on poi with a revolver than an auto.
I imagine that must depend on manufacturer and caliber (and a lot of tradition, I bet).
I’m pretty sure that e.g. a heavy .22LR revolver isn’t going to have much recoil effect compared to a light 10mm automatic, right?
I would not be surprised to find that “generic revolver”, to the extent the term has any meaning at all in the industry, is sighted for closer ranges than “generic automatic”, even if there’s no really sensible reason for it.
A small correction. The bullet accelerates as it travels through the barrel from zero at ignition to muzzle velocity as it exits. This acceleration is what causes recoil. As a result, travel time in the barrel is roughly double what is shown in your sample calculation.
Next time I’m adding a line: “math has been greatly simplified so even a small child can understand it.”
For a first approximation, the only variable that matters is the mass of the bullet. Accelerating the same bullet faster rotates the gun faster but the bullet also leaves the barrel earlier. These two effectively cancel out. The other variables are gun- and grip-specific and don’t change with the load.
More precisely, the only thing that matters is chamber pressure and time.
Since both loads are likely loaded so SAAMI pressures, the force on the bullet, and the equal and opposite force on the rear of the cartridge case are equal to the chamber pressure multiplied by the area of the rear of the case.
Making a few simplifying assumptions–chamber pressure remains constant during in-barrel bullet motion (reasonable, given pistol powder burn rates, and sufficient for this excercise), the relative time spend in the barrel will be the square root of the relative mass. Since the rearward force during this time is approximately the same, the overall impulse will scale accordingly.
So the the first approximation mentioned by Dave is correct, but misleading–it is time that matters in recoil impulse, as the recoil force is an effect of pressure from the gas on the rear, the reason bullet mass matters is only through its effect on the time over which the pressure is applied.
Interestingly enough, there is an additional FORWARD force during this time–the drag of the bullet on the rifling, but this can be ignored for this approximation.
So much for “let’s keep it simple.”
Well, it isn’t simple, so there is that.
The physics behind the simple explanation may not be simple, but they’re also irrelevant.
The explanation of why heavy bullets hit higher is straightforward. You don’t need to know the physics to know why. I’ll leave the math to the nerds.
Why not just say aim a tad lower with a heavier bullet. It saves me a ton of money in Advil.
One of the major reasons this effect is noticed more with revolvers vs. semi-autos is that the moment arm that translates recoil into angular momentum around a horizontal axis, is usually much larger in revolvers than in semi-autos. This is the “grip” effect mentioned above. If the barrel axis line is such that there is no moment arm, there is no such effect.
Revolvers that shoot out of the bottom of the cylinder would minimize this effect, because they minimize the moment arm.
Contrary to the first sentence in the article, the energy used by an auto-loaders has minimal contribution to this effect, as nearly all of it occurs after the bullet has left the barrel.
A good approximation of the moment arm would be the distance from a line along the center of the barrel to a line that extends from the center of the grip along the arm toward the shoulder. The distance should be measured at the grip, The distance would be measured perpendicular to the axis of the barrel.
Ah, I remember when my boss at the gun store explained this to me when I asked him why swiching from 158gr JHPs to 125gr JHPs chnged the POI in my .357 so noticeably. Calculating muzzle jump is what makes calibrating sights so fun for engineers. . .
Interestingly enough, this was considered vital enough information that how muzzle jump works and what effect it has on sighting was considered critical information for British conscripts in basic training. . . at the height of WWII.
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