For a semi-automatic or bolt-action weapon, AIUI the barrel is sealed all the way from the brass cartridge to the muzzle (except for whatever gaps may exist because of rifling in the barrel), so all of the propellant gas is working to drive the projectile forward.
For a revolver, there is some sort of clearance gap between the inlet end of the barrel and the outlet end of the multi-chambered cylinder holding the round (necessarily to allow the cylinder to revolve). Some of the propellant gas must escape through this gap instead of driving the projectile down the barrel.
How much difference does it make? If I have a revolver with a certain total length from the back of the cylinder (where the brass cartridge seats) to the muzzle, and a semi-auto of the same caliber with a barrel of that same total length, how do the muzzle velocities of these two guns differ?
There will be some escape, but not enough to make much difference. Some designs move the cylinder forward to seal the gap, but most designers didn’t think the added mechanical complication was worth it.
Depending on the design of the revolver, somewhere from 5% to 30% of the propellant gas may be lost through the cylinder gap. General opinion seems to be that the bullet has usually already left the barrel by the time most gas escapes, but a couple of articles in NRA publications suggest that the Nagant revolver improved velocity by almost 20% due to its gas seal mechanism. (The Nagant was a bit unusual in the overall design of the mechanism and the cartridge.)
It’s … there, but a part of a whole bunch of factors when it comes to firearms - type of ammo and especially barrel length and pressure can make a difference as well. If you want an idea (not direct) here’s a link where they did tests with a multiplicity of factors in which they used variable gaps to try to get an idea. Full of Excel based goodness if you like.
I read an article years ago noting that everyone speculated that the revolver would shoot slower because of the cylinder gap so the writer put it to the test with two cartridges (I think 9mm and .45ACP). He found that the revolver was very slightly faster. He speculated at the time that even though the revolver lost some pressure at the cylinder gap, it had a longer effective barrel length because the autopistol barrel is measured including the chamber whereas the revolver barrel length is measured after the cylinder chamber. The author speculated that the extra effective length of the revolver more than made up for the loss of pressure at the cylinder gap.
This guy talks about running the same kind of test in similar conditions and getting the same result. Maybe it’s the same guy and his informal research has been widely and frequently repeated. It would be a cheap to run yourself if you were interested.
Well, one obvious difference is safety measures. A revolver has hot high pressure gasses coming out near the hands. My understanding a la youtube is it can cause burns or injury if you don’t put your hands in the correct spot.
If you wanted to experiment and check the speeds, I think you would also have to take into account barrel tolerances (friction between bullet and barrel, bullet bouncing its way down a comparatively wide barrel), rifling vs smoothbore, whether you can get identical ammo in revolver and pistol form factors, the part of recoil that cycles the bolt on pistol (probably other effects like recoil in general and gun moving because recoil don’t have major effect)
I’m pretty sure either mythbusters or youtube have highspeed of revolvers firing, so it should be possible to check whether gases come out before or after the bullet leaves the gun
The hardest part might be controlling for all the variables.
I mean, if there was some hypothetical gun with the same barrel that came in revolver, bolt-action and semi-automatic, then we could potentially actually get some data.
But nothing like that exists for all three that I’m aware of- usually you have two of the three in any given round- e.g. semi-auto vs. bolt action .308 Winchester rifles, or revolver/semi-auto 45 ACP (even if we’re talking half-moon clips in a revolver). But even there, they’re different barrels- length, rifling, etc…
My deduction is that all else being equal, the muzzle velocity will shake out like this from fastest to slowest:
Bolt action
Semi Auto (assuming gas-operation)
Revolver.
The reasoning is that for the same barrel length, the bolt-action will have higher pressure for longer than the other two. And the semi-auto mostly loses pressure toward the end of the barrel where the gas is tapped to cycle the action. Finally, the revolver loses pressure right at the beginning of the barrel, meaning that it’ll have lower pressure than the max possible for the entire length of the barrel.
But that’s assuming gas semi-automatic operation; if you have a recoil-operated semi-auto, it shouldn’t give up any velocity to a bolt action, as the pressure should be identical, and the action only cycles once the bullet has left the barrel.
Basically IMO, there’s no way to know for sure, but the thought experiment above shows you one way to think about it.
Wanted to dig up some additional numbers for you, best (other than the Ballistics by the Inch I linked earlier) was an article from 2016, which included some very detailed shim based estimates of power loss by gap size. Concluding “You might say that this roughly averages as a loss of 10 foot per second velocity for every one thousandth of an inch extra cylinder gap.” Admittedly, that was one caliber, in one gun, but should give a rule of thumb.
A weird point I want to bring up due to my fondness of PCC (Pistol Caliber Carbines) is that the average legacy revolver round (357 and 44 mag) have a lot more power than they can show out of a typical pistol length barrel. At least one trial showed between a 40-50% increase in velocity between a 4" pistol and an 18" carbine barrel. I’m sure the better seal in the lever action makes a difference as well, but by comparison shooting a 9mm PCC only gains around 10-20% velocity between a a 4" and a 16" barrel. So revolver rounds are apparently (and we’re still leaving out a ton of ammunition weight/charge/+P variables) overpowered for the platforms they’re shot from.
Since the choice of revolver vs semi-auto/bolt normally has a lot more to do with the planned usage of any firearm, I feel the tradeoffs in velocity are normally not a consideration for the average buyer. But hopefully we’ve given at least a bit of factual information towards the consideration.
If you lose a slight amount of velocity, from an engineering standpojnt rather than try to make a revolver with a gas-tight cylinder (or a finely mchines cylinder that lets out much less gas, It is easier to just use a slightly more powerful cartridge.
Just don’t wrap your hand around the cylinder when you pull the trigger. Much damage will ensue.