Velocity and Barrel Length

[Snakebite]

If you have a load of say 50grs of FFF shooting a .495 RB out of a 34" .50cal barrel at 1200fps, does it take more or less powder to achieve the same 1200fps velocity out of a 44" barrel using the same ball/patch combination?

 

[don hepler]

Someone might have actual chronograph data, but my guess is that it would take a little more. I think I read that a black powder load would reach something like 80% of it's pressure in maybe 20 inches. So if that's true, at some point it would actually be dragging in the bore, and slowing down. But someone, probably has documented data.

 

[galamb]

You can have a look at a test that Rice barrels did.

Using 80 grains of 2F they started with a 44" barrel, fired test shots (chronographed), chopped 2" off the barrel/re-crowned, shot again and so on down the line.

The results for the 34 and 36" barrels are transposed when you look at the chart.

The bottom line was/is, you get better velocity, with the same charge out of a longer barrel.

That could be read conversely, you should get an equal velocity, from a longer barrel with less powder than a shorter barrel would require.

Here's the Rice tests (Rice tests)

(I personally don't care what the velocity is - hitting the target accurately is far more important - likewise, Bambi doesn't seem to have noticed whether a PRB traversed his lungs going 1200 or 1800 fps but we all tend to be obsessed with numbers and charts)

 

[hanshi]

Generally, the longer the barrel, the more velocity from a given powder charge. After about 30" to 36" each inch of length tends to give less and less extra speed.

 

[George T.]

That is a great experiment. I never would think that there would only be 100 FPS delta with a foot less barrel between 32 and 44 inches. I wonder what difference more powder and thicker patches or even 3F would alter the results.

However I realize that you could shoot for years to meet all combinations.

Geo. T.

 

[Jim Evans]

So the most consistence of the 4 shots was the 38"and the 30",the least consistence was the 34" and 44" barrel, when you look at the extreme spread.

 

[Rifleman1776]

So the most consistence of the 4 shots was the 38"and the 30",the least consistence was the 34" and 44" barrel, when you look at the extreme spread.

A lot of things in the shooting world do not seem to make sense. This is true in modern guns as well as our antique styles.

 

[mooman76]

There are still a number of unknown variables in the gun world that don't make since because we just don't know all the facts.

 

[galamb]

I wouldn't read a great deal into this single experiment.

If you upped the powder by 5 grains, or went with a .018" patch or .495 ball I'm positive the numbers would look somewhat to totally different.

I think Jason did this test just to show that there is "some" difference depending on the length of a given barrel (all else being equal).

The bottom line that can be drawn is, as long as you are using enough powder to overcome the friction of the barrel, all else being equal, a longer barrel will result in somewhat more velocity.

But that doesn't mean I want a 60" barrel just to wring out an extra couple hundred feet per second when I could add maybe 10 or 20 grains of powder and get the same speed (which I don't care about anyhow).

 

[hanshi]

A lot of things in the shooting world do not seem to make sense. This is true in modern guns as well as our antique styles.[/quote]



Very true. In my .40 a 5 grain powder increase going from 35grns to 40grns gave a velocity jump of 256fps, a huge jump. Didn't do anything close to that at any other charge level.

 

[eaglesnester]

sometimes when u add more powder u do not get an increase in velocity. at least that is true for sure in smokeless powder center fire cartridges.