Feet per second with longer barrell??


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Northslope Nimrod
September 15, 2005, 01:37 AM
How many feet per second does a 24" barrell provide over a 20" barrell in a 30-30 shooting a 170 grain bullet?

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50 Shooter
September 15, 2005, 01:40 AM
I think the standard for any rifle barrel is 25-30fps per inch. So you're looking at 100-120fps difference.

dakotasin
September 15, 2005, 08:58 AM
i haven't chronographed the 30-30, but i would bet that the gains/losses on a 30-30 would be most insignificant.

anonanon
September 17, 2005, 12:07 PM
The variation in individual guns is greater than the velocity difference betwen a 20" and 24" barrel. For example, going way down to where barrel length does make a difference, my 6" .357 chrono's a particular load at 1400 f/s, my 8" only 1300 f/s, and my 6.5" 1325 f/s -- and that's averaging out a lot of rounds too, not just one off.
The only way to measure velocity difference would be to cut back a barrel a couple of inches at a time. Someone did that once, starting I believe with 28", then hacksawing the barrel down to 26", and so forth. Sorry I don't remember the caliber; could have been anything from .22LR to .30-06, and the results do not necessarily hold true for other calibers. But I remember it was a pretty consistent 50 f/s each two inches, right down to 16"...which ties in with the figures mentioned above.

Strongbad
September 17, 2005, 06:36 PM
The difference in feet per second gains per inches of barrel length are going to hinge a lot on what powder you're using, which is typically dictated by the catridge itself. Cartridges which use slower powders will typically pick up more speed per inch of extra barrel than those that use faster powders. Dan Lilja was the guy that did the expirement that the other gentlmen mentioned (or he's one of the guys that has done that), you should be able to find it on Dan's website. www.riflebarrels.com I know that loads out of my 338/378 with a 29" barrel generally run about 200-300 fps. faster than guns with standard 26" barrels. Since the 30/30 uses powders that are significantly quicker than the 338/378 (RL15 vs. RL25) you're not going to pick up as much as I am. In the case of your gun, I'd be looking for the barrrel length that points best for you for that gun. :) Good luck.

anonanon
September 17, 2005, 10:16 PM
Strongbad -- Thanks for the reference to the velocity/length data. (Wasn't the article I read and remember badly.) It is excellent authoritative data you can count on.

Archie
September 19, 2005, 01:05 AM
1. Ratio of powder to bullet weight, the more powder you burn for any specific bullet, the fast the bullet goes. (Duh...)

2. 'Expansion Ratio', the ratio of volume of the cartridge case versus the volume of the case and the interior of the bore where the bullet leaves the barrel. Which is why a longer barrel - in general - shoots faster than a shorter one. Anonanon, I'm sure you realize cutting your 8" barreled gun won't make it shoot as fast as your 6" gun, right?

The expansion ratio expands faster in larger bore guns. So adding two or three inches to a .35 caliber rifle means more than adding two or three inches to a .22 caliber rifle.

Do a search on line for "Powley Computer". Homer Powley was a ballistician in the 1950s and 1960s who developed some mathematics to predict the interior ballistics of normal rifle rounds. The original computer was a cardboard sliderule thingie. Just playing with it and seeing how various combinations 'work' teaches one a lot about the subject. I think there's a place on line where you can play with a 'virtual' mechanism.

sumpnz
September 22, 2005, 01:09 AM
I remember reading, I think on Varmint Al's site, a test where the took a .17HMR barrel and started around 26" and cut it back 1" at a time to 14" or 16" shooting I think 10 shots at each length. Velocity peaked at 18-19". Obviosuly by 19" the pressure from the propellant gasses was no longer high enough to overcome friction in the bore which caused the MV to drop as the barrel got longer.

As far as .30-30 goes, I really don't know what the barrel length would be for max velocity with typical 170gr factory loads. In the end though, Stronbad's suggestion of looking for the length that points most naturally for you is probably more important than the 50-150fps you might gain with a longer vs shorter barrel.

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