Revolver vs Auto velocities

[mountaindrew]

I came upon this idea while I was looking into velocities for my 9mm sp101 revolver and I dont know why I have never heard anyone question it before:

Why is it that a auto's barrel length is measured from the back of the chamber and a revolvers barrel length is measued from the front of the cylinder? in a 2in revolver, the bullet has as much actual tube ( whether or not it is "barrel" is in question) to accelerate in as a 4in auto. I know the barrel gap in a revolver vents some pressure but is seems unfair to compare velocities when the testing methods are so different. Thoughts?

 

[WT]

A few years ago SWAT magazine did a study comparing muzzle velocities from a 9mm Ruger revolver with a 9mm Ruger auto. Ruger happened to be a company that made firearms in 9mm with the distance from the breech to the muzzle being the same.

Anyway, some ammo actually had a higher muzzle velocity coming out of the revolver as opposed to the auto. Not all types of ammo tested, just some. SWAT was pretty surprised.

Unfortunately I can't remember the reason for the difference.

 

[RWK]

This question was discussed with some frequency on TFL. In essence, an autoloader’s effective barrel length is reduced by the fact that the cartridge is seated in the barrel, thereby reducing the bullet’s “travel†by the approximate length of the cartridge. Although a revolver employs the full barrel length (plus, perhaps, some cylinder length), there is a pressure (thus velocity) loss through the cylinder gap. Most of the data presented on TFL, as I recall, suggested marginally greater velocity from revolvers, all conditions being equal.

 

[zaijian]

I think I read somewhere that at the moment the powder is ignited, the force of the gases causes the casing to momentarily expand, eliminating the cylinder gap, which effectively seals it.

 

[jc2]

but is seems unfair to compare velocities when the testing methods are so different. Thoughts?

Who says life is fair? Just know that they are different and get over it.

I think I read somewhere that at the moment the powder is ignited, the force of the gases causes the casing to momentarily expand, eliminating the cylinder gap, which effectively seals it.

NOT HARDLY! If you have access to revolver try hold a piece a paper (NOT your hand) beside the front of the cylinder when you touch one off.

 

[JNewell]

He's crossing the breech end of the cylinder with the cylinder/barrel gap. The case does expand to seal the breech but this is true of all conventional fixed ammunition, whatever the action type.

 

[stans]

Perhaps a revolver might have slightly higher velocity since the bullet initially accelerates through the unrifled cylinder throat, little or no friction to impede its acceleration before engaging the barrel as opposed the the semi-auto bullet engaging the rifling almost immediately. This is just my theory.

 

[Archie]

Once again....

Holster makers were the ones who decided how to measure barrel lengths. It's just a way of keeping track of how long to make the holster.

Ballisticians measure barrel length from the base of the bullet], whereever that may be, to the muzzle. That is the effective distance the propellant gasses are pushing on the bullet.

Cylinder gaps do vent some pressure. However, the velocity loss is ignorable in most cases. There is more variation in different revolvers and pistols than in a generic loss between autos and revolvers.
Just last week I shoot some reloaded ammo in two different .357 Magnum revolvers. This ammo was full charge 158 grain SWC bullet-2400 powder stuff. The revolvers were a 7.5" Ruger Blackhawk and a 4" S&W 581. The 581 had the higher average velocity for 12 rounds fired. But only by about 10-15 fps. I don't have a .357 pistol, so I can't compare.
I also shoot some 45 ACP hardball duplication ammo in two different pistols; a 1911 (Remington UMC) and a Colt commercial Series 70 Government Model. The Colt gave roughly 25-50 more fps in 10 round averages. I didn't take my 1917 revolver to compare.

The belief that autos in general give higher velocities is unverified by experiment. As part of an ongoing experiment, is there anyone in the greater Los Angeles area with a .45 ACP revolver who would care to meet me at San Gabriel Valley Range and chronograph some loads? I have the chrono and several auto pistols in 45 ACP; and one revolver. I need a greater cross sampling of revolvers. I'll even supply the ammo.

 

[James Bondrock]

I think I read somewhere that at the moment the powder is ignited, the force of the gases causes the casing to momentarily expand, eliminating the cylinder gap, which effectively seals it.

That would only be true if it were a Nagant revolver.

I recall a magazine article in which the author compared a Ruger convertible single action revolver in 9mm with a 4 5/8" barrel to a Browning Hi-Power with a 4 3/4" barrel. He could not understand why the Ruger produced higher velocities given the same length of barrel. The answer is that with the Ruger or any revolver, the entire length of the barrel is actual rifled bore (minus about a quarter inch of forcing cone), while you must subtract the length of the chamber from the effective barrel length of the pistol. The extra 3/4" of bore produced most of the velocity gain, the loss through the cylinder gap notwithstanding.

 

[grendelbane]

It is too small a sample, but I have chronographed several .45 Gov't models and a couple of 1917 revolvers with the same load.

All came reasonably close to 840 fps. The wheelguns had more barrel to work with.

Of course, the auto loses a little bit of velocity from the rearward motion of the slide. A fixed barrel with no cylinder gap would be the most efficient.

I remember being mildly shocked at the high velocities that a cheap .32 ACP derringer produced compared to some pistols that had a moderately longer barrel length.

I recently acquired one of the old .45 Colt/.410 Contender barrels. With the choke removed, it is roughtly equivalent in length to my 6 1/2" M29. I want to chrono it, and see how well the old .45 does in a single shot. With Silvertips, it seemed to thump pretty good, appearing much hotter than any wheelgun I have fired with the same load.

 

[WheelMan]

I don't think there's any velocity loss from the slide recoiling, not measurable anyway.

My strange velocity story involves .40S&W. I don't have the numbers in front of me but my 3.9 inch (rated) desert eagle produces nearly identical velocities to my 4inch 646. While the 646 has an advantage in barrel length (.1 inch plus however long a .40 case is). The revolver has that gap and the eagle has a polygonal barrel (which should speed things up a bit). Interestingly the 4.5 inch .40 barrel that I had for my witness clocked about 50fps slower than the other .40s, must have been "tight" or something.

 

[grendelbane]

Yes, there is a loss in velocity from the slide recoiling. You could lock the slide to the frame, and actually measure it. For every action, there is an equal, and opposite reaction. The bullet and the slide are moving away from each other. Of course, the bullet is moving much faster than the slide.

Using my imperfect memory, I think that the slide velocity on a Gov't model pistol is about 23 FPS. Some loads will produce that much shot to shot variation. Still, it is enough to be measured, even if it is insignificant.

Some barrels do indeed seem to be "fast", or "slow". I have a SIG P226 that I consider slow. I have an old Colt Official Police that I consider to be fast. Many things can affect this, including smoothness of the bore, rifling style, wear, cylinder gap, etc.

 

[WheelMan]

I'm pretty sure the bullet is out of the gun entirely before the slide comes back any. Ever seen a slow motion video of an automatic firing? Even if it did it doesn't matter. IF you put a gun down on a table and could somehow set it off (NOBODY DO THAT) you would observe the same projectile velocity when the peice goes skittering off the table as you would when it's securely in your hands.

 

[grendelbane]

Only in a non-Newtonian universe would this occur. The slide and the bullet both start moving at the same time. The bullet moves much faster, however. This is why you can not detect the motion with your eyes. It is still there, however.

Think about it. Jump just as high as you can. No one can detect, or measure how far you pushed the Earth away, but you did. Your movement, relative to the Earth was much greater, and was measurable.

If your pistol weighed exactly what your bullet did, (not considering the weight & velocity of the powder charge, for simplicity), then it would recoil towards you with the same velocity.

Probably the easiest to measure situation with actual firearms would be a blow back semi-auto .22, fired with the bolt free to move, and with the bolt locked to the receiver of the gun. The velocity would be measurably higher than when the bolt was free to move.

It is not a tremendous difference, however.

 

[WheelMan]

I think we agree.

I'm an engineer, not a scientist. I tend to pretend that differences I can't detect don't exsist.

 

[grendelbane]

I am neither a scientist or an engineer, nor do I play either one on TV. It took me a while to get to the total understanding of the movements involved in something as simple as an automatic pistol.

Now, consider the Benelli shotgun, with the Montefeltro action. If it were in orbit, touching nothing else, and fired?

Would the action operate, and feed the next shell into the chamber?

The opposite side of that coin is welding the receiver to a massive chunk of steel, and firing it. Will the action operate in either circumstance?

I think that the answer is no in either case.

 

[WheelMan]

I'm not familar with how that shotty works, but if it's a recoil based action I guess it would come down to the intertial mass of the moving parts of the action versus the non-moving parts. You'd want high non-moving mass and low moving mass. Weak springs would help too.

 

[JNewell]

Can't be much loss due to the slide's action. Unless it's a blowback action, the breech is locked until after the bullet leaves the barrel.

 

[grendelbane]

It is not a great deal of lost velocity, but it is measurable. In a Gov't model a typical bullet weighs 230 grains. Slide/ barrel weight will vary, but let us pluck a number out of the air and say it weighs 8765 grains, (about 1 1/4 pounds).

That means the slide/barrel weigh over 38 times what the bullet does. (I am leaving out the weight of the recoil spring, and ignoring the existence of the powder charge).

In a 5" barrel, the nose of the bullet is about 3 3/4" from the muzzle. (This distance should be measured from the muzzle to the base of the bullet, but I can't remember how long the bullet is.)

Anyway, the bullet moves forward about 38 times faster than the slide moves backwards. The bullet's average speed in the barrel is about 420 FPS. (It accelerated from zero to 840).

The slide accelerated from zero to about 23 FPS in the same time.

So the slide did not have time to move very far before the bullet left the barrel, but it did move. Had the slide and barrel been locked to the frame, a small , insignificant increase in velocity would have occurred.