Effects of barrel length

[Homesteader]

We all know there's quite a difference in performance when it comes to barrel lengths of BP revolvers. I'm curious as to the specifics. Col Colt seemed to like around 8" for his, though I've read there were plenty of "belt" pistols that were closer to 5".

Can anyone here give actual info regarding the relative merits of barrel lengths? Not sighting, but the ballistics / lethality involved?

(Zonie, I'm glancing in your direction! :wink: )

 

[tenngun]

For sure you will add velocity, and could make use of the biggest powder charges. Having a short barrel back then added to ease to carry. Most people who carried a belly gun back then depended hitting from just out of arms length where 50fps wouldn't matter. Longer "duleling" guns came rise to hand gun target shooting. A few folks like Hickock and Okley could shoot the eye out of a knat on the wing. Most people shot a belly gun at less then ten feet.

 

[Zonie]

Your just trying to get me in trouble. I can see it all now.

Darned if I know off the top of my head.
The best I can do is to look at my Lyman Blackpowder Handbook & Loading Manual (2nd Ed.).

Seems their .36 caliber revolver just reaffirms what we would guess about how the barrel length would effect velocity.

They tested a .36 cal 5 inch and 7 inch revolver using a .357 diameter ball using GOEX 3Fg and got the following:

5 Inch Barrel

15 grain = 618 fps
17.5 grain = 685 fps
20 grain = 750 fps
25 grain = 819 fps
27.5 grain = 847 fps

7 1/2 Inch Barrel

15 grain = 668 fps
17.5 grain = 742 fps
20 grain = 794 fps
25 grain = 843 fps
27.5 grain = 868 fps

Change In Velocity

15 grain = 50 fps
17.5 grain = 57 fps
20 grain = 44 fps
25 grain = 24 fps
27.5 grain = 21 fps

No. I don't have any idea why the gain seems to go down as the powder load goes up.

My logic would say it should be the other way around because in theory the smaller powder load should burn itself out rapidly (like before the ball leaves the muzzle?) while the larger powder load should be producing power throughout the length of the barrel and the extra length of the barrel should allow this larger powder load to accelerate the ball more, not less.

I guess that just goes to show, logic doesn't always predict what happens in the real world. :hmm:

 

[robert reed]

No. I don't have any idea why the gain seems to go down as the powder load goes up.

Possibly, definitely don't know, a larger charge, taking longer to burn out, starts blowing out the cylinder gap. The fire would naturally feed on the oxygen source and may start to reverse the direction of the gas expansion. Or start sucking power back from the projectile.

All this would be happening so fast, I don't know how this could be tested. :hmm:

 

[Chowmif16]

Could also be the law of diminishing returns. As the velocity of the ball increases due to the increase in powder charge, so does the drag on the ball in the barrel from friction. As I recall, that friction increase is exponential with velocity. (Hope I'm right...)
Might be that its getting to the point near 800-900 fops in which the drag really starts to take effect.
As with all these things, it's probably an interaction of several things. I like the comment about leakage from the cylinder gap too. Higher pressure = more likelihood of pressure loss from the cylinder gap.

 

[M. De Land]

Wonder if the same dynamic would occur with 2F instead of 3F?

 

[Zonie]

Lyman didn't test any 2fg powder in their revolvers so it's anyones guess (unless on of our members with a .36 with a 5" barrel and one with a 7 inch barrel and a cronometer wants to burn a lot of powder and time to test it).

Just for giggles, here's the data for the two guns using Elephant 3Fg powder:

5 Inch Barrel

15 grain = 542 fps
17.5 grain = 574 fps
20 grain = 648 fps
25 grain = 692 fps
27.5 grain = 738 fps

7 1/2 Inch Barrel

15 grain = 566 fps
17.5 grain = 638 fps
20 grain = 666 fps
25 grain = 745 fps
27.5 grain = 792 fps

Change In Velocity

15 grain = 24 fps
17.5 grain = 64 fps
20 grain = 18 fps
25 grain = 53 fps
27.5 grain = 54 fps

Now, ain't that a fine kettle of fish? :grin:

About the only thing I see that seems to make sense is the Elephant powder is producing velocities about 100 fps slower than the same amount of GOEX. :grin:

I do have to add that the velocity difference is more like I would expect with the longer barrel gaining more velocity with the larger loads. Well, except for that strange 20 grain powder load.
Maybe the longer barreled gun got an extra dose of bark charcoal instead of wood charcoal in its mixings?

 

[smoothshooter]

I suspect Colt's preference for 7 1/2" to 8" may have been chosen at least in part because of simple mechanics. Those lengths are a good compromise because they allow the loading lever to be just long to make seating the oversize balls in the chamber mouths relatively easy. Anything shorter is noticeably harder to seat balls with.

 

[Loyalist Dave]

We should also note that there is gas loss in the cylinder gap....it would be interesting to chronograph single shot pistols of various barrel lengths....

LD

 

[nkbj]

Reckon the chamber diameter makes as much difference as a couple more inches of barrel?

 

[smoothshooter]

Most of the time, the barrel / cylinder gap loss is little more then the shot - to - shot variation in a shot string, assuming the gap is less than about .005". Also, remember that on a single shot, barrel length is measured from muzzle to breech which includes the chamber area whereas on a revolver the chamber is not included in the length measurement. A 5" revolver barrel may give equal or greater velocities than a 5" barrelled single shot of the same caliber.

 

[Grumpa]

You are right on target. :thumbsup:

 

[Grumpa]

Does Lyman say how many shots were chronographed with each load? 1, 10, 20, 100?

Did they use a 7 1/2" bbl, and then cut it back to 5" for the comparison (so we're comparing the same bbl, with lengths being the only variable)?
Or did they (more likely) use 2 guns, which is really comparing apples and oranges)?

How did they account (if they did) for consistency in loading? That alone could account for some of the differences.

All of these factors ( and more ) have an effect on muzzle velocity. With so many variables, a backyard approach is hardly scientific, and of limited use.

What I get from the Lyman figures is that the more powder loaded, the more velocity gained - but not necessarily in proportion; and, as you get to the higher charges, less velocity is lost in the shorter barrel.

But I would want to chronograph my own loads, in my own guns, over time, to find out what is really going on.

Zonie, none of my comments are directed at you, I just have questions about how Lyman arrived at the figures. They may have used a scientific approach, or just a casual, hobbyist approach - which is too often seen in many firearms articles.

Your post, as usual, is interesting and thought provoking. :hatsoff: (And maybe I am getting carried away - it's been known to happen.

:haha: :haha: :haha:

 

[Zonie]

Lyman used "5 shot test strings" for all of their data. I assume they averaged them to get the numbers.

I really doubt that they got out the old saw and started whacking off barrels. They don't say but their data looks like they used different guns if the barrel length was different.

It's long been recognized that increasing powder loads increases velocity but the amount of gain decreases as the load increases.

Below is a graph showing this in a .45 caliber rifle.
Also note that there is a point where the rise in velocity changes. This is used by some to guide those who want to get the best gain at the least expense. Others who are just looking for high velocity don't give a hoot about the amount of powder needed for max velocity.

The irony is if the load is for a roundball, by the time the ball gets out to 100 yards, most of the big gain has disappeared.



PS: Hopefully when your getting "carried away" your still breathing. At our age, that's very important. :rotf:

 

[Col. Batguano]

What are the charge increments that each of those boxes represent?

It's interesting at the upper charges of pyrodex, the larger granulated powder produced higher velocities than the smaller stuff. Not so with the Goex though. Interesting.

 

[Zonie]

The boxes at the lower left are for 40 grain powder charges.
Each box to the right of those lower boxes represents a load increase of 10 grains with the upper right hand boxes being 120 grains.

Put another way, starting from the lower left on any given type of powder graph the values of each box is, 40, 50, 60, 70, 80, 90, 100, 120 grains.

The velocities were achieved in a 28" long barrel, with a rifling twist of 1:48, shooting a .440 diameter roundball patched with a pre-lubricated "Hodgdon Pyro" .010 thick patch.

 

[Blizzard of '93]

revolvers do not have a patched ball so there is some blow-by of powder charge gas past the ball/slug. does a larger charge of powder result in more blow-by? and more velocity inconsistency? I'd say so as group size begins opening up. at least in my experience. sometimes though the velocity gain from heavier powder charge is deemed neccesary enough to forgo group tightness - as a hunting charge for example.
in a front-stuffer loaded with a PRB stout charges can 'blow the patch' which I'd reckon would result in great variation in velocity and likely accuracy. shot-to-shot that is. and may not be desired for anything but relatively close range shots on some large game animals.
intersting topic for sure.

 

[Grumpa]

revolvers do not have a patched ball so there is some blow-by of powder charge gas past the ball/slug.

Actually, when using a soft, over-sized lead ball that has been rammed into the cylinder, so as to cut a ring of lead, there is no blow-by of gases within the cylinder. The lead is a much more efficient sealer than a lubed patch.

There is a minimal loss of gas at the cylinder/ barrel gap (see smoothshooter above).

If the revolver is properly set up, the ball, once in the barrel, will continue to seal the gas off until exiting the muzzle. This can be clearly seen in high speed photos, where the nose of the bullet is emerging prior to any gas. The gas emerges only after the bullet is out.

If groups start opening up at higher velocities, you might want to try a larger ball (say a .454 or even a .457 in a .44 caliber revolver). That will give you a wider bearing band for an even better seal.