Truth be told on barrel lengths


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Sig Bill
May 15, 2012, 08:05 PM
I just found this out and it confused me.

When the barrel on the revolver is measured, it's from the forcing cone to the muzzle. There you have the "speaking" length, the length that the bullet travel.

Now on the autoloader the chamber is included in the measurement and that threw me off. The chamber is not part of the "speaking" length of the bullet travel. I thought it was from the head space to the muzzle.

So while my Sigma 9mm have a 4" barrel I don't really have a 4" barrel, I have 3.25" of the "speaking" length of the barrel.

WTH?

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The Lone Haranguer
May 15, 2012, 08:58 PM
Autoloader barrels, unlike revolvers, have a chamber integral with the barrel. Therefore they are measured from the rear of the ejection port to the muzzle. So, yes, the actual length of rifled bore in a 4-inch barrel is about 3 1/4 inches. It is just the standard for how they're measured.

Carl N. Brown
May 15, 2012, 09:08 PM
To use a phrase a lot of people hate, it is what it is. Or it is what it has been forever.

Barrel length on selbstgeladen pistolen and pistols like the Thompson Center single shot is from muzzle to breech face and includes the firing chamber: the length of the part you would buy as a "barrel".

Barrel length on a revolver is the length of the part designated as the "barrel" which is forcing cone to muzzle, and the firing chamber is part of the cylinder, a seperate part.

I used to argue with people that in comparing barrel length on a revolver to an automatic you needed to count the length of the cylinder to get a better comparison.

My nightstand gun is a snub .38, barrel length is 2 1/16" but the measurement from muzzle to breechface is 3 5/8" (the cylinder is 1 9/16").

epijunkie67
May 15, 2012, 10:14 PM
I used to argue with people that in comparing barrel length on a revolver to an automatic you needed to count the length of the cylinder to get a better comparison.

I would have thought the reverse would be a better measure. Instead of adding the length of the revolver cylinder I would think you should subtract the length of the chamber on an automatic.

Adding cylinder length gives you an accurate primer to end of barrel measurement. But what really matters is the distance the bullet is going to be traveling. And that should be measured from the starting point of the bullet, not the primer.

Unfortunately nobody in the industry bothered to call either of us before they set these standards so we're both gonna have to keep doing math in our head whenever we compare pistols. LOL

jmr40
May 16, 2012, 09:19 PM
Neither is a fair way to compare. The 2 are very different and while it may seem confusing things are probably best left as is. If you have a 4" revolver and a 4" semi auto firing the same ammo you will still get more speed from the semi auto even though the barrel is technically shorter. (Assuming you want to include the length of the cylinder in the barrel length of a revolver). Revolvers lose a lot of pressure and velocity when the bullet jumps the gap between the front of the cylinder into the barrel. Not taking the length of the cylinder into account on revolvers is probably the fairest way to make peformance seem equal between the two.

Caliper_RWVA
May 16, 2012, 09:46 PM
I agree it is somewhat misleading in terms of comparison.

However, practical considerations rule here. It is the way that is easiest to measure on each. In both cases, if you order a "barrel", the part that comes will be the length listed without having to subtract chamber length or add cylinder length.

Carl N. Brown
May 17, 2012, 10:26 AM
Personally I don't think a 2.5" revolver barrel with a 1.5" cylinder would lose 1.5" worth of velocity compared to a 4" barrel auto.

Looking around, more than one source, including:

http://www.thehighroad.org/archive/index.php/t-105593.html

Cylinder gap can run .002" (too tight to be practical unless you want to clean frequently), Ruger single actions run .003" to .004" new. A lot of different double actions .006" new. Used revolvers can run up to .010" and still be servicable.

The gas comes out that cylinder gap with a lot of pressure (just shoot a revolver with a tent of folded cardboard rested over the top of the gun). Mythbusters found the flash from the cylinder gap of a .500S&W can decapitate chicken wings. But the loss of total pressure behind the bullet is not huge.

A wide cylinder gap can cost 100fps, typical loss about 25fps to 50fps velocity. Some lots of ammo can vary by that amount round to round from the same box.

Individual revolvers and automatics with equivalent breech-to-muzzle length, same ammo, give different results based on bullet size and barrel fit. Generally, the auto will be marginally faster.

mgmorden
May 17, 2012, 10:32 AM
To me, its a bit of a non-issue. Barrel length factors into two things for me:

1. Velocity that can be developed, and
2. Sight radius.

Revolvers lose a bit of gases due to the cylinder gap which makes them a little less capable of generating the same velocities of an auto with equal rifled barrel lenght.

Regarding sight radius, autos typically need *at least* the length of a cartridge behind the barrel in blank space to facilitate the reciprocating action of feeding cartridges from the magazine. That puts the rear sights that much further back. As a result, an auto with 2" of rifled barrel surface would have much greater sight radius than a revolver with the same.

All in all, when those are taken into account, the difference in measuring kinda cancels out. Even though measured differently, an auto with a stated length of 4" has a sight radius similar to a revolver with the same stated barrel length.

mj246
May 17, 2012, 07:17 PM
As a result, an auto with 2" of rifled barrel surface would have much greater sight radius than a revolver with the same.
--mgmorden

How do you come to that conclusion?

It's not like revolver rear sights are in front of the cylinder either. Both revolvers and semi-autos have the rear sights at the back of the frame/slide respectively and the front sights near the front end of the barrel. I can't imagine a significant difference in sight radius between the two if:

1) same length of rifled barrel,
2) chambered for the same round (approximately equal chamber lengths), and
3) similar frame size (can't really compare a snubby N-frame to a sub-compact semi even if they were chambered for the same round, which none are to my knowledge).

A semi-auto of equal OVERALL length to a given revolver would likely have a slightly longer sight radius due to a revolver's grips generally extending back from the frame and a semi's extending down, but it would still usually be less than 0.5" difference. That is not really a lot in terms of sight radius IMHO.

EDIT TO ADD: Just measured, and if the overall length is the same, yes the semi-auto will have a significantly longer sight-radius (about 1"-1.5"), but still confused on the aspect of measured barrel length giving more sight radius to a semi-auto. I have a revolver with about the same (measured) actual barrel length (not including chambers or cylinders) as my brother's 9mm 1911, and my revolver has a LONGER sight radius (but also a significantly longer overall length). My other revolver has a similar overall length to his 1911, but a shorter sight radius. So again, I'd say it's based more off of overall length, not just barrel length.

Warp
May 17, 2012, 10:02 PM
Neither is a fair way to compare. The 2 are very different and while it may seem confusing things are probably best left as is. If you have a 4" revolver and a 4" semi auto firing the same ammo you will still get more speed from the semi auto even though the barrel is technically shorter. (Assuming you want to include the length of the cylinder in the barrel length of a revolver). Revolvers lose a lot of pressure and velocity when the bullet jumps the gap between the front of the cylinder into the barrel. Not taking the length of the cylinder into account on revolvers is probably the fairest way to make peformance seem equal between the two.

Good post.

JRH6856
May 17, 2012, 10:24 PM
FWIW, if anyone is using QuickLoad software, QL subtracts the case length from the barrel length to calculate the distance the bullet travels before exiting the muzzle. The calculation is the same for rifle, pistol or revolver, and while correct for pistols, a 6" barrel .357 ends up with a calculate bullet travel of 5.1 inches. With the cylinder included in the barrel length, the calculated travel is more like 6.8 which is closer to correct.

R.W.Dale
May 17, 2012, 10:52 PM
I own and have been chronographing various loads from a 2" snubby chambered for 9mm

The most surprising thing I've discovered is how given the same loadings I see velocities similar to my glock 34

For example from this 2" supposed bbl 124g speer gold dot +p leaves the muzzle at close to 1160 fps.

This is only 60 fps less than speers numbers given for a 4" test barrel

So yes the cylinder of a revolver does in fact act as effectivly a bbl extension

mgmorden
May 18, 2012, 03:27 AM
mj246: You're overthinking it. It's not a hard fast rule - even different guns of the same barrel length may measure out slightly differently

Consider this: if the revolver has 4" of barrel (not including the cylinder), then the front sight will generally be right at the end of that. Now you go back the distance of the cylinder (which is about the length of a cartridge) and the rear sights will generally be at the rear of that plus a little extra depending on frame design.

Now, if a semi-auto has a stated barrel length of 4", then the actual rifled barrel length might be less, but once you get to the end of the 4" including chamber, you then have to have an additional space behind the chamber in slide length of at least 1 cartridge so that the slide can move back and forth and the action works. You'll have to go at least that distance to make it to the rear sights.

With that in mind, it won't be exact (and the actual length of the cartridge itself plays a role), but in general, a 4" revolver (measured without the cylinders) will have a sight radius in the same ballpark as a 4" semi-auto (which includes the chamber).

45_auto
May 18, 2012, 07:18 AM
Revolver barrels aren't always measured from the forcing cone, sometimes they include the cylinder.

The ATF considers the barrel length to be the distance between the end of the barrel and the breech-face.

If you want to build an SBR (short barreled rifle) on something like the Colt Revolving Rifle or the Rossi Circuit Judge, the barrel length will include the cylinder.

From the ATF:

http://www.atf.gov/publications/download/p/atf-p-5320-8/atf-p-5320-8-chapter-2.pdf

The ATF procedure for measuring barrel length is to measure from the closed bolt (or breech-face) to the furthermost end of the barrel or permanently attached muzzle device. Permanent methods of attachment include full-fusion gas or electric steel-seam welding, high-temperature (1100F) silver soldering, or blind pinning with the pin head welded over. Barrels are measured by inserting a dowel rod into the barrel until the rod stops against the bolt or breech-face. The rod is then marked at the furthermost end of the barrel or permanently attached muzzle device, withdrawn from the barrel, and measured.

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