Semi-Auto and Revo Barrel Lengths

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Feb 15, 2004
The Gunshine State==FL
Why do semi-auto manufacturers include the chamber length when determining their pistols barrel length? Case in point is my Kahr MK9 with an advertised barrel length of 3.0" but the actual rifled barrel is only approximately 2.0". A S&W Model 337 revolver I once owned had an advertised barrel length of 1.9" which is exactly how long the rifled portion of the barrel was.
Because barrel length can determine velocity and energy, shouldn't semi-auto pistol makers be obliged to advertise the actual rifled barrel length?
I realize that some "pressure" is lost at the gap between a revolver's cylinder and barrel but isn't that about equal to the amount of "pressure" lost when the semi-auto slide is racked back at the moment of firing? Both of these actions must have some effect on the guns velocity/energy especially in a short barreled piece, albeit very slight I agree.
When you get to be a retired old curmudgeon and have too much time on your hands you get to think about things like this and I know that some of you guys out there will come up with an answer.:cool:
I have always assumed just for ease of measuring. Now, by convention. If you are trying to compare straight across (like, .38 vs. 9mm) don't forget this, but its not going to change.

Also (to get all nitpicky, like we do around here), revolvers have some leade/forcing cone area, so the functionally rifled bit is shorter than the measured barrel length. A similar thing is on all guns with chambers attached to the barrel. There is some space between the chamber mouth and the rifling.
Not being a died in the wool revolver man, please correct me if I'm wrong, isn't the forcing cone on a revolver in the cylinder and not the barrel? If not what is the proper terminology for that area inside the bore at the front of the cylinder that is smaller in diameter than the rear part of the bore?
Nope, the forcing cone is the unrifled cone-shaped lead in area of the barrel in a revolver. The straight walled and smaller diameter front area of each cylinder is called the cylinder throat.
The problem is that the ATF and other legal authorities use the different measurements to enforce laws - and this is a major PITA. If one adopts a common standard, one could drop a rod down the barrel until it touches the breechface, mark the point where it emerges from the muzzle, and measure that length to give a "barrel length". This is done in autopistols, and so includes the chamber measurement. However, in revolvers, the cylinder is not included, so that measurement is from the forcing cone to the muzzle. This means that in, for example, Canada, where there is a legal minimum handgun barrel length of (I think) 120mm., or just below 5", a pistol can make this minimum length with 4" or less of rifled barrel, while a revolver has to have a full 5" barrel plus cylinder length, making it much less concealable and carryable.

Makes no sense at all, does it?

:fire: :banghead: :confused:
The general consensus is that losses due to the barrel/cylinder gap in a revolver compensate for the fact that it has a longer effective barrel length resulting from the different measurement conventions.

There is no pressure loss in a semi-auto due to slide motion.
The actual length of the barrel when the length is measured. Where the round chambers is beside the point.
Unfortunately this backward way of measuring bbl lenghts really put a crimp on revolver owners for a 4" is 101mm & according to Cdn law one must have a bbl length of 105mm or over. Anything under that is PROHIBITED.

Okay for us that have 2" to 4" & guns that were bought prior to '91 but try to sell a 4" & you have a lot of people turning it down since they do NOT have "Prohibited" on their Possession & Acquisition License.

Also another "prohibited" are calibers 32 or under bar 22 rim fire so a lot of European semi-autos cought there.
barrel length

In a single shot or self loader the barrel extends all the way to the standing breech. Drop a rod down and measure it.

In a revolver, stick a feeler gauge through the cylinder gap, drop a rod down the barrel and measure it. You have the lenght of the BARREL. Note that the cylinder is NOT part of the barrel.

These measurements are intended as the actual barrel length and not for the purpose of estimating performance or losses due to cylinder gap, operating the action, etc.

If you measure from standing breech to muzzle in all types of guns, the single shot is the simple case. For the same distance, a revolver typically lose around 50 to 100 fps due to the gap and a recoil operated pistol will lose something on the order of 10 to 20 fps operating the action. A blow back operated action is a good deal messier when it comes to calculating the losses but they are on a par with the recoil operated action which is the simplest of the three to calculate the loss for.
I don't think everyone agrees with the ATF method. Its good and reproducable, but if you look at the specs gun makers publish, is not right. We sat down and measured some stuff once, and sometimes they would count the end of the chamber hood, sometimes run under and count some other cut out, and sometimes they just didn't match at all by like 1/4". So, that's another issue entirely.

The -- somewhat limited -- amount of comparing of velocities makes me seriously doubt that you loose that much speed due to cylinder gap. I can't recall the gymnastics involved, but have heard arguments that revolvers of a given length are /faster/ than autoloaders. Whatever.
Velocity loss

It's relatively easy to calculate the velocity loss involved in operating a recoil operated action to a fair degree of accuracy.

It's more difficult to determine the loss in a blowback and harder still the loss due to cylinder gap. You inevitably end up comparing apples and oranges.
SO WHAT if revolver "A" with a four-inch barrel sends a 125-grain JHP downrange at 1500 fps and autoloader "B" a four-inch barrel sends a 125-grain JHP downrange at 1350 fps?

What is important is what each individual weapon will do--not how the barrel length is measured (unless you are just looking for something about which to argue).

The barrels are measured diffently--big deal.
So, you don't like the way they are measured. Get over it!

Technically, the barrel of the revolver IS measured (the cylinder is NOT part of the barrel), and the barrel of the autoloader IS measured (the chamber IS part of the barrel).

Barrel length may or may not effect the velocity and energy. I've seen shorter barrels that shooter "faster" than longer barrels (with the same load). The bottom line is that ALL that really counts is what the particular load will do in your particular weapon. Stop sweating the small stuff.
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I don't know if JC2 is retired yet but he certainly has achieved curmudgeon status.
JC2 if a member's questions bother you that much, just click out and go someplace else. Your attitude sux!
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