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Old June 2, 2006, 01:49 PM   #1
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Barrel Length vs. Muzzle Velocity

Lots of manufacturers list the muzzle velocity (and other ballistic data) for their ammo. And some of them specify the length of the test barrel used to develop the data (for example Federal uses a 24" test barrel for many of its hunting loads).

My question is this:
Is there reliable rule of thumb that will tell me the reduction or loss of muzzle velocity if I use a shorter barrel than the manufacturer's test barrel (say, for example, my barrel is 18" or 20" long and the ammo manufacturer tests with a 24" barrel)??

p.s. I know the best way would be for me to use a chronograph and test the ammo, but I don't have one.

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Old June 2, 2006, 02:16 PM   #2
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I've understood that you loose about 50 fps per inch of barrel for each inch less than the test barrel, but I don't have any official source or calculator.
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Old June 2, 2006, 04:04 PM   #3
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A friends Timberwolf carbine in .357 did about 1800 fps

or so. This checks out with Twycross and his estimation of 50 fps per inch of barrel.
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Old June 3, 2006, 12:03 AM   #4
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I've got a few rifles in 7mm/08. Two of them are 22 inch barrels and one is 18.5. I find that there is usually about 80 fps difference between them, so about 20 fps/inch. Of course every rifle is a rule unto itself so other rifles and other cartridges could vary quite a bit.

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Old June 3, 2006, 07:55 AM   #5
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Seen and read numerous tests over the years...Most enlightening one was where they took a heavy barrelled .308, started with a 26" barrel, and then shortened and recrowned it 2' at a time, down to 18" (5 different lengths).

They were losing about 150 fps for each 2" of barrel removed. (Side note: accuracy remained unchanged).

This lines up pretty well with other things I've read and heard, that say something like 50-75 fps per inch of barrel length. Variables include cartridge (duh), powder burn rates and bullet type.

I suspect that, if you handload, you could minimize the effect using a relatively fast powder, for more "burn" in the shorter barrel. Would be an interesting experiment
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Old June 24, 2006, 07:02 PM   #6
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adding barrel

Is there any data for adding length of barrel. I shoot a Browning High Wall in 45.70 with a 28 inch barrel. Most of the data I have seen for projected speed of amunition in that caliber is for 22 to 24 inch barrels. I would be curious to see how much a longer barrel would increase velocity.
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Old June 24, 2006, 07:11 PM   #7
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Probably a stupid question, but does the rate of twist have anything to do with velocity? I know it's for stability but just wondering..

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Old June 24, 2006, 07:35 PM   #8
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Technically, the faster the rate of twist, the faster the round SHOULD be

BUT, I think the difference is nominal unless you go from 1 in 14 to 1 in 7. Obviously, a change that big not good unless your shooting small caliber (5.56). I don't think it makes a REAL difference, maybe an average of 10 fps if you went from 1 in 9 to 1 in 7 in an AR. Before anyone beats me over the head with this, it is purely speculation. I have no proof to back up the numbers, I'm just doing something that will probably make me look dumb and speculating.
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Old June 24, 2006, 08:03 PM   #9
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That 50FPS/inch generality is probably pretty good. Out of my old 2.5" Smith 66, .357 Magnums were supposed to be good for about 1000FPS, but the same loads out of my Marlin 1894C (18.5" barrel) should be good for around 1800FPS.
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Old June 24, 2006, 08:05 PM   #10
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Because pressure curves are not linear, acceleration rates down barrels are not constant numbers of feet per second per inch. It comes close to linearity in the last inches of a long barrel, though. A velocity vs. distance down the tube graph from QuickLOAD is provided below for the .45-70. The bullet is 405 grain Remington soft point over 4198 loaded to SAMMI maximum pressure. The blue line is the velocity (scale on right edge), while the red line is pressure (scale on left edge). Inches of barrel is the X-axis along the bottom.

As a point of information, SAMMI specifies a standard test barrel and test barrel length for each chambering. For example, for .308 Winchester, it is a 24 inch barrel with a 12" Right Hand twist, 4-groove rifling with grooves 0.176+0.002,-0.000 inches wide, and a copper crusher piston hole 0.206" wide. If a SAMMI member manufacturer specifies a velocity for .308 ammunition without specifying the barrel length, you may assume it was shot in one of these.


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Old June 25, 2006, 01:04 AM   #11
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Rule of thumb

A rough rule of thumb is 35-50 fps per inch for handguns, and 50-75fps per inch for rifles, BUT it is only an approximate. Each gun is a law unto itself.

I have seen a 100fps difference between three different guns with the same barrel length firing the same ammo. This is a little outside of the average, but it does happen. So it is possible for a "fast" carbine to equal the velocity from a "slow" rifle length barrel. The only sure way to tell for certain is to chronograph your gun/ammo. Anything else is just an estimate.
All else being equal (and it almost never is) bigger bullets tend to work better.
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Old June 25, 2006, 04:16 AM   #12
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You're all forgeting something ! The loss per inch of barrel depends on the initial velocity. The higher the intitial velocity the more is lost when you cut the barrel .A velocity of 3500 fps will be reduced significantly when you cut the barrel a few inches . A 45-70 velocity reduction will be negligible. I cut my 1885 Browning , 45-70 ,from 28" to 22" and didn't worry about it knowing the losses would be insignificant !!
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Old June 25, 2006, 09:10 AM   #13
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I pretty much go along with dfaugh. About every ten years or so, somebody goes to barrel chopping and checking the velocity.

Generally, it runs about 75 ft/sec/inch for cartridges with cases like the '06. Stuff like the .308 might lose around 50 ft/sec/inch. Magnums run closer to 100 ft/sec/inch.

Years ago, 26" was the standard for published velocities of factory ammo. With the advent of all the reloading component suppliers' data books, other lengths have become common. Still, the ft/sec/inch numbers hold reasonably true for horseback guesstimates.

Maximus, the twist rate is more a function of the ratio of length of bullet to its diameter. Longer bullets (heavier) need faster twists than short, light-weight bullets.

Lessee, what else? Faster burn-rate powders in short barrels mostly just reduce muzzle flash. A max load in a short barrel won't give any gain in velocity over the slow-burn powder in that barrel; just easier on the eyebones at night.

Drifting: One barrel-chop test of a 10" .44 Mag revolver started at around 1,600 ft/sec with 250-grain bullets. Down at one inch of barrel, the muzzle velocity was around 1,150 ft/sec. Out of curiosity, they removed the remaining piece of barrel and chronographed the last shot. Amazingly, it was right at 1,100 ft/sec. That certainly would make an interesting carry gun.

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Old June 25, 2006, 09:29 PM   #14
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One thing to consider is that as the bore diameter increases (larger calibers), there tends to be less benefit from a longer barrel.

I think this is because as the bullet moves down the barrel, the larger caliber means that the volume of bore behind the bullet increases much faster than it would with a smaller caliber. The result is that the expanding gases have to fill a much larger area and that reduces the pressure much more rapidly as the bullet moves down the bore. Less pressure means that the bullet isn't accelerated as rapidly as it normally would be--at least after the first few inches of barrel length.

Obviously, you can just design the cartridge to hold a lot more powder to compensate for this effect (as with the .50 BMG) but if you stay in the same general neighborhood in terms of power, expect to see less benefit from a longer barrel with larger bore diameters.

Sorta goes along with the results of Art's posted example.
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Old March 2, 2013, 12:55 AM   #15
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25 fps loss per inch of barrel lost

This paper shows the result on velocity and loudness of the report of shortening a rifle barrel:
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Old March 2, 2013, 01:06 AM   #16
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Since I last posted on this thread, about six and a half years ago, I have become aware of a very useful resource that speaks to this issue...

These folks have spent a tremendous amount of time (and money) testing different calibers (mostly pistol calibers) in different length barrels.
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Old March 2, 2013, 12:10 PM   #17
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There are lots of factors and it is almost impossible to say for sure. Most common barrel lengths are 20-24". With anything in between those ranges I've found 15-25 fps/inch to be most accurate. Actually the most accurate method I've found, (and it is still not great), is to predict velocity change to be somewhere between .5% up to 1% for each inch of barrel length change.

The only numbers that really mean anything when doing research is to find someone who has chronographed a load from a longer barrel, cut the barrel, then chronographed it again with the same ammo. There are way too many other factors when you compare velocities from different guns. Around 50 fps difference between guns with equal barrel lengths with the same ammo is common and I've seen as much as 130 fps difference. I've also seen 20" guns shoot faster than 22" guns and 22" guns shoot faster than 24" guns.

As you start cuttting barrels shorter the velocity drops off in larger increments. A barrel cut from 20" down to 16" will likely lose a lot more than when the same barrel was cut from 24" down to 20". The chambering matters too. Rounds like the 308 are more efficient than magnum rounds, but no matter how short you cut them, the magnums will always be faster from equal barrel lenghts. I often read incorrect comments like " if I were going to shoot a 22" barreled 300 mag, I'd just as soon shoot a 308".

With most guns there is very little to be gained from barrels longer than 24". With all guns there isn't enough to worry about with only 2" difference. If you are deciding between a gun, even a magnum round, with a 24" vs 22" barrel there are more important things to influence your decision.

Here is another good read. The author tested multiple rifles by chronographing, cutting and chronographing again at different barrel lengths. Here are his results.
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Old March 2, 2013, 06:55 PM   #18
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Here's the results of SAAMI's tests using the same barrel and ammo and cutting off in inch at a time. Different cartridges were used to get data in different muzzle velocity ranges.

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