'relative velocity' numbers; how much variation in different barrel lengths?

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kmrcstintn

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my particular question deals with revolvers, hence 'vented' barrel relative numbers given by manufacturers; "roughly" how many fps are gained with each added inch (eg: 125gr .357 mag load measured @ 1450 fps from 4" vented test barrel has how much more velocity from a revolver with a 6" barrel) and "roughly" how many fps are lost with each inch less in length (eg: 200gr .45 colt load measured @1000 fps with 7.25" vented test barrel has how much less velocity from a revolver with a 4.75" barrel); I do not have access to a chronograph and I want to get an estimate of speed from different loads in different calibers from different guns; thanks
 
There is an excellent website that deals with this. They actually took Contenders of different calibers and progressively cut the barrels off inch by inch and measured the velocities of several factory loads at each length. I can't remember the exact name of the website now. Something like "ballistics by the inch" I think. The guy that did it has posted here so do a search on what I just said. It is fascinating research. My overall impression is that the factories do a pretty good job of pushing different barrel lengths for different calibers. As I recall calibers like 44 mag show a huge increase with each inch of barrel out to around 8-9" or so, then the velocity gain is fairly small. Same thing with something like 38 special for instance. It gained a lot up to about 5-6" or so then kind of tapered off. I don't remember them all exactly.

I'll see if I can find the site.
 
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What exactly is proved when revolver ammunition (like 38 Special +P) is fired through what appears to be an unvented single barrel pistol? (Well, yeah, I know it shows what the velocity of the ammo is if it's fired from an unvented single barrel pistol - but that's all it proves.) I fire my revolver ammo from a revolver; complete with inconvenient cylinder gap gas leaks and all.

The velocity statistics that are shown in which real world guns were used may be reasonably accurate - except that I wouldn't trust any test very far that reports results for 38 Special +P ammo as results for "38 Special".

Nevertheless, the question that the OP asked was a very good one - and IMHO a very important one, because every LEO out there needs to know what the trade-offs are that he's making when he's chooses to use a particular barrel length; just like he should know the trade-offs he's making when he chooses a particular bullet type. I'm not sure anyone even knows the answer to this question right now because - well, let's take revolver ammo specifically: It's usually tested (and the velocities are reported by manufacturers) using "vented barrels; not actually real world revolvers. Maybe there's no difference, but if not, why not use real revolvers? I'd like to see every manufacturer of handgun ammunition -particularly manufacturers of revolver ammunition - be required to report velocities of the bullets they manufacture using certain commonly used real world guns for testing, not 4" vented barrels, and I'd like to see them show (on their boxes) the different velocity a particular round of their ammo acheives using several different barrel lengths -say, 2"; 4"; 6" and 8" lengths; i.e., sort of a truth in ammo advertising law.
 
What exactly is proved when revolver ammunition (like 38 Special +P) is fired through what appears to be an unvented single barrel pistol?
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In this case it proves that it's a lot easier and cheaper to replace a Thompson Contender barrel than it is to replace a revolver barrel. :D

Ok, seriously. Given that they provide a LOT of comparison velocities in a LOT of different guns (with vented & unvented barrels and a variety of barrel lengths) using the tested ammunition, it shouldn't be too hard to get a feel for how those numbers would apply to a revolver.
...except that I wouldn't trust any test very far that reports results for 38 Special +P ammo as results for "38 Special".
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But... But... How do you know it's .38 Special +P ammo? (He asked rhetorically.) Ahhh, yes, because the information is right there on the page with the reported results so that readers can see exactly what kind of ammunition was used. ;)
Maybe there's no difference, but if not, why not use real revolvers?
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Primarily because it's possible to come up with a standard vented barrel for use with testing that will guarantee uniform numbers regardless of who is doing the testing. When you purchase ammunition you want to know how it compares to other ammunition and having all manufacturers use a standardized test barrel will give you that information.

Trying to have every manufacturer use a real world gun introduces a lot of variables into the situation and that would make it difficult (if not impossible) to compare the figures from one manufacturer with the figures from another manufacturer. Unless of course, every manufacturer tested their ammunition in all common guns and reported all the results--and I think we all know why that's not being done...

What you can be sure of is that the published numbers put out by ammunition manufacturers are going to be representative. The manufacturers really have no choice given that every gun writer and many ordinary shooters have chronographs these days. Any manufacturer not reporting representative numbers is going to get a lot of negative feedback in a hurry.
I'd like to see them show (on their boxes) the different velocity a particular round of their ammo acheives using several different barrel lengths -say, 2"; 4"; 6" and 8" lengths;
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I think that having manufacturers provide this information (though perhaps not directly on the boxes) would be a great idea.
 
JohnKSa:
Ok, seriously. Given that they provide a LOT of comparison velocities in a LOT of different guns (with vented & unvented barrels and a variety of barrel lengths) using the tested ammunition, it shouldn't be too hard to get a feel for how those numbers would apply to a revolver.
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Yup, that's why and how we did it. Thanks, John. The data and documentation is all there for everyone to draw their own conclusions.

MADISON, if you look at the actual data, you'll see that it is nowhere near that simple. I have completely given up on such "rule of thumb" calculations.

Jim D.
 
The old rule of thumb

Was 25-50fps per inch. However, that is only a rough estimate, and may not apply to any specific gun. Some guns are faster than others. Some slower.

I once chronographed a 100fps difference (high to low) between 3 different 6inch .357 magnums, firing the same ammo. This is uncommon (the large spread), but not impossibly rare.

My advice is to use any and all published data as guidelines, and expect your gun/ammo combination to be somewhere close, but by no means spot on.
In other words, take the velocity figures with a grain of salt, until/unless you chronograph the ammo out of your personal gun.
 
Obviously my eyes are failing me, because I'm being told by one respondent to this thread that something is printed in the 38 Special section of ballistics by the inch that I still don't see, and that's any notation that they're talking about 38 Special +P ammo; not standard velocity 38 Special. The way I realized it was probably +P they were talking about was not by reading anything that said it was it was; that notation isn't there to read. The results just sounded too hot to be standard velocity 38 Special ammo, although - and this is my point exactly - who could know whether these numbers were too hot to be standard velocity ammo or not (without being told) since the ammo in question was not fired from a revolver but from what appeared to be an unvented single barrel TC in which there would have been no cylinder gap gas loss? But the ammo in the accompanying picture was 38 Special +P so I assumed thats what was fired in the tests. Or was it? Who knows?

Please note that in these tests, there were 45 ACP results reported as +P so maybe everything else is correct, I don't know. I didn't read beyond the 38 Special data (except to verify that there were 45 ACP rounds reported as +P ) but none of the 38 Special data was reported as +P.
 
Sorry about that--I thought you were being snarky about the fact that they included photographs of the ammunition boxes showing the +P designation rather than typing in the information somewhere. :(
 
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