fps vs barrel length?

aheppner

aheppner

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how many feet per second can you expect to gain with every inch of barrel more then a published test barrel speed

iv been using 69 smk with 24 grains of imr 4320 with a 30'' barrel 2923fps

the hodgdon data shows 23 grains 2673 and 24.8 grs 2873 with 24'' barrel

i thought i should be a little faster?

with my math at 24 grains should be 2858.5 with a 24in barrel
and i am at 2923

so there is only a difference of 64.5 fps from a 24'' to a 30'' barrel


it just occurred to be that my twist rate is different, the test rifle is 1-12 and im 1-7, would that account for the low speed gain?
 
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Best way to find out is with a chronograph, don't believe the crap about barrel length, there are a lot of variables at work.
It may tend to be faster with a longer barrel, but there is no hard and fast rule. It may even be slower in your rifle, as it won't be the same specs as the test barrel.
Different propellants react differently to barrel length changes and a host of other things.
 
Longer barrels only provide faster speeds if the powder is still burning in them. Short barrels may not burn up all the powder in the given tube. A lot of stuff factors in so no real true rule to go by
 
i did chronograph my load, it was faster than book speed but not by much,

i heard some where you could get 30 to 50 fps per inch, if your using the right powder

i was 64 fps faster than a 24'' barrel and i have 6'' more barrel, but i do have a faster twist so i guess that slows it down a bit


im going to be loading up some 80 amaxs with varget soon, ill see what i get with that and compare
 
aheppner said:
i did chronograph my load, it was faster than book speed but not by much,

i heard some where you could get 30 to 50 fps per inch, if your using the right powder

i was 64 fps faster than a 24'' barrel and i have 6'' more barrel, but i do have a faster twist so i guess that slows it down a bit


im going to be loading up some 80 amaxs with varget soon, ill see what i get with that and compare
Click to expand...

Twist rate should not affect speed.
 
Each barrel / chamber is different. When you compare your barrel to theirs there are a lot more differences than just the barrel length.

I once had a 32" barrel I wanted to shorten a lot, so I took a hacksaw and cut it off 1" at a time. Velocity dropped about 25 fps per inch. It was a 308.
 
In my 20.5 inch takedown 308 i am faster than my Icon P.H 308 with 22 inch barrel with Superformance 165 gr SST 2805 for the 20 incher and 2795 for the 22 incher so... get a Chrony... JP.
 
Ganderite said:
Each barrel / chamber is different. When you compare your barrel to theirs there are a lot more differences than just the barrel length.

I once had a 32" barrel I wanted to shorten a lot, so I took a hacksaw and cut it off 1" at a time. Velocity dropped about 25 fps per inch. It was a 308.
Click to expand...

thats a cool test, thats exactly what i wanted to know
 
John Y Cannuck said:
Best way to find out is with a chronograph, don't believe the crap about barrel length, there are a lot of variables at work.
It may tend to be faster with a longer barrel, but there is no hard and fast rule. It may even be slower in your rifle, as it won't be the same specs as the test barrel.
Different propellants react differently to barrel length changes and a host of other things.
Click to expand...

Exactly. Altitude, for instance. You would be lucky to start off with results exactly as shown in a reloading manual.
 
I would imagine that after a certain length, once the power is consumed, there will be diminishing returns. I could also imagine that with a longer than neccisary barrel you will actually slow the projectile down. Drag, friction etc.

Cheers!
 
aheppner said:
are you sure, cuz it seems to me that pushing something through a tube with no twist would be alot easier than one with a tight twist
Click to expand...

It doesn't work quite that way.
A molycoated, or greased bullet, will be slower than a regular bullet.
 
At one time in my past, I owned two Ruger #1 rifles chambered in 30-06.
One was a #1B with a 26" tube, the other a #1AB, with a 22" tube.
They had throat lengths that were basically identical in length [long, lol]
Both barrels slugged identical inside dimensions, [.3084"]
With 59 grains of IMR 4831 behind the 180 Partition, the shorter barrel was almost 75 fsp FASTER than the longer barrel. Figure that one out!!
There are a lot of factors involved in the speed of a bullet exiting a given barrel.
As has been suggested...a chronograph is your friend.
Regards, Eagleye.
 
I have read that twist will affect speed. As will many other things. I know twist will make a gun pull to the side in the really big bores so less is better if you can stabilize the bullet.
 
toebako said:
I have read that twist will affect speed. As will many other things. I know twist will make a gun pull to the side in the really big bores so less is better if you can stabilize the bullet.
Click to expand...

Never heard that one before.
 
I have had the opportunity to be a Gun Nut for the last 60 years and have either worked in ballistics labs or had access to fine labs most of that time.

This has meant a lot of scientific testing. One of the keys to good testing is the limiting of variables, so you know what you are looking at. The problem with testing is that each barrel is different, even though test barrels are made only by the best barrel makers to very strict uniform standards. I would say that the variation from test barrel to test barrel is about 100fps and 5,000 psi.

When you start comparing YOUR rifle to someone else's (the lab barrel or your buddies) the variation is even greater. And some variations are huge. Chamber and barrel dimensions vary more than you might think.

So when you wonder about the effect of twist on pressure and think because in your experience, rifle A has higher velocity than rifle B and the reason must be the twist, I have to smile.

In order to make that test, one needs at least 10 barrels, identical except for twist. As it happens, this has been done. (As has just about every test you can imagine.)

Some factory barrels are rifled using a sine bar. This makes it easy to vary the twist from barrel to barrel. As i recall the test, 10 consecutive barrels were rifled, with the twist adjusted between each barrel, from a normal twist to no twist. All barrels got the standard factory chamber.

When tested, there was no apparent pattern to the pressure (velocity) differences as one would expect to see if twist had an impact.

It is my observation that the big variable is throat depth and diameter. I have used 308 barrels as tight as 0.3065".
 
Rifle Magazine and Rifleshooter Magazine have published excellent articles on this topic. I recall at least two good ones and there is probably more. I wish I still had them. I am sure you can purchase or download a copy.
 
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