Velocity VS. Barrel length

[RU shooter]

I know with regular loads using rifle powders there is an increase in vel. as the barrel length increases example being 20" vs. 26" barrel.Now does this also happen with loads using the faster pistol and shotgun powders like Red dot, Unique and 2400 with normal weight cast boolits?


Tim

[JeffinNZ]

Quite possibly you lose FPS in a longer barrel. Case in point, I have a .40cal MLer the barrel on which was originally 39 inches. I chopped the barrel to 33 inches and FPS on light charges of 25gr went UP 150fps and the top end load of 40gr, the FPS stay the SAME.

[Bass Ackward]

More variables are case capacity and bore diameter. In a larger bore, pressure drops faster making barrel length less a factor as pressure is less of a factor.

[RU shooter]

More variables are case capacity and bore diameter. In a larger bore, pressure drops faster making barrel length less a factor as pressure is less of a factor. I was wondering mainly about 308/30-06 and similar 30-31 caliber rifles with the loads being in the 20K through mid 30K psi. range.

[MtGun44]

Bass is talking about expansion ratio. A straight case with a 2" long case with 26" barrel
expands the initial volume by 12 times (26-2=24" working length divided by 2" original
case length) because the bore is approx the same diam as the case.

Take a .220 Swift. Without calculating anything you can see that the bore volume is small
relative to the case volume for the same length of each. Maybe around 3-4 times more, so with
a 2" case and 26" barrel the expansion ratio would be 3-4 times less than the straight
case. So the expansion ratio would be approx between 3 and 4 compared to 12 in the
straight case.

A .220 swift can therefore take advantage of a longer barrel a lot more than a .22 LR or
.45-70. The .45-70 really doesn't need more than about 16-18" (exp ratio ~ 7-8) and
a .22 LR won't need more than about 12-14" to get good velocity, ballpark 1" case so
a 12" bbl gives 11x exp ratio. I have read that .22 LRs actually slow down in the bbl
if it is over something like 16". No proof, tho.

So - depends on the cartridge (expansion ratio) and the powder speed.

Bill

[felix]

In the 30-06 the demarcation is 16.5 inches with experiments using only one powder (4895?), one barrel continuously chopped. The ACCELERATION of the expansion was seen to reverse direction at this inch setting. The does not mean the velocity afforded by the remaining acceleration did not increase velocity. This was determined experimentally by bore interrogation (rotational junk on the lands) after each barrel chop.

Another article in the Rifleman talked about chopping a revolter in like manner. The conclusion was a 4 inch barrel is the optimum balance with the calibers tested with nominal factory type performance loads.

... felix

[Mk42gunner]

Barrel length (and condition of the rifling) can definately affect velocity and expansion of bullets. The most striking example that I ever experienced happened to my Dad and I. One day we were plinking, using hedge apples for reaactive targets. I was using his Winchester Model 67A with 27" barrel, he was using his Winchester Model 190 with 20" barrel. Both using ammo from the same box, probably Wildcats. The ones I hit just had a hole drilled thru them; the ones Dad hit exploded in chunks.

Two possibilities for the results: The longer barrel had slowed the bullet enough that it didn't expand as fast as the shorter barrel; or the rifling was sharper on the newer gun, prestressing the bullet so it deformed easier.

I don't have Dad's 190 so I can't make direct velocity comparisons, but may try with some of the other .22's I have around.

Robert

[felix]

Excellent observation, Robert! ... felix

[missionary5155]

Good morning
I think it was shooting times that ran several "Tests" with barrel lenghts some years ago. One test was a 7.5" Ruger 44 mag that got lopped of 1" at a wack. depending on the three powers used (fast , medium, slower pistol powders) Barrel length was very interesting in affecting speed. Another test was with a .22 rimfire using a couple of barrel blanks welded end on end. Somewhere around 8 feet the boolit did not exit the barrel. Somewhere aboy thwt 16" mark was the best velocity with a couple of brands. The HYPER velocity stuff fared better with about 19"
Did someone do a 308 Win test with a 32" barrel blank ? One of them old military rifles with a shot out bore would be an interesting candidate to lop off using maybe 3 burning rates of powder...

[felix]

I think I remember that article. That article was solely after peak velocity statistics. Very interesting stuff to me, all of this "expansion" stuff. Always looking for a better, more consistent gasoline and sparkplug to manipulate exquisite timing for volumetric applications. ... felix

[Larry Gibson]

I was wondering mainly about 308/30-06 and similar 30-31 caliber rifles with the loads being in the 20K through mid 30K psi. range.

In that psi range you will get higher velocity with the longer barrels using normal powders such as Unique and slower burning powders. However, it is possible with some heavier bullets using a fast burner such as Bullseye to attain that level of peak psi and get a slower velocity out of the longer barrel. You would really have to work at it to accomplish such. I have done so with the .308W and the '06 with 18/26 and 20/24" barrels using a 311299 with Bullseye in "cat's sneeze" type loads. If I recall correctly I also found the same with 140 gr cast bullets in the 6.5 Swede using a M38 and M96 with cat's sneeze loads. However i was not pressure testing then so I don't know if the psi was up into the 20 - 30 psi range.

Larry Gibson