The Myth of Barrel Length and Velocity Loss

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Dave P

New member
It's not a myth: if you use a slow powder, it will burn longer while developing more velocity. So a long barrel can produce more velocity if you use a slower, but suitable powder.

This is easy to see in pistol loads: a fast powder will tend to have no, or reduced muzzle flash. A slow powder will generate a big ball of flame at the muzzle, because the powder is still burning after the bullet exits the barrel.
 
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Double Naught Spy

New member
Right, not a myth. So what I get from the article is that because Sharpe found velocity loss was an average of 12 fps instead of the 25 fps per inch most people thought and therefore velocity loss is a myth. I don't see how anybody could possibly reach that conclusion. The provided information just said it was less than expected, not that it was wrong or a myth. The title is misrepresentative. If anything, the documented loss proves velocity loss is a fact.

I think this article is a bit more informative. The purpose wasn't to document velocity loss from barrel shortening, but to see if accuracy was affected. They measured accuracy and also velocity. The barrel was cut down by 2" at a pop. The lost 350 fps over the course of 8" (an average of 43.75 fps per inch) and lost 55 fps per inch or more in the middle range.
http://findarticles.com/p/articles/mi_m0BQY/is_1_50/ai_110470564/
 
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Webleymkv

New member
A variety of factors influence velocity and barrel length is just one of them. Powder burn rate, bullet weight, powder charge, and atmospheric conditions also play a role.
 
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jmr40

New member
If folks bother to do 5 minutes of research on google they can find lots of data to debunk most of the myths of barrel length and velocity. Barrel length does have an effect on velocity. Just not nearly as much as many old wives tales that keep getting repeated would have us believe.

I've done a lot of testing, and research and have foune that the most reliable indicator is to plan on somewhere between .5%-1% of velocity change for each inch barrel length changes. This means that if you are getting 3000 fps from a barrel, cutting it 1" shorter will change velocity somewhere between 15-30 fps.
 
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kraigwy

New member
It's not a myth. Velocity does change with barrel length. How much depends on the ammunition, caliber and gun.

Here is an example. My granddaughter did a science project showing something else but she needed to know what changes to velocity occurred.

She started out with a 26 inch 308 barrel, and Remington factory 180 grn corlok bullets.

The first column is barrel length the second is velocity (at 10 ft from muzzle.)

26 2826
24 2722
22 2713
20 2677
18 2654
16 2552
Click to expand...
 
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jmr40

New member
It's not a myth. Velocity does change with barrel length. How much depends on the ammunition, caliber and gun.

Here is an example. My granddaughter did a science project showing something else but she needed to know what changes to velocity occurred.

She started out with a 26 inch 308 barrel, and Remington factory 180 grn corlok bullets.

The first column is barrel length the second is velocity (at 10 ft from muzzle.)


Quote:
26 2826
24 2722
22 2713
20 2677
18 2654
16 2552

__________________
Click to expand...

Her data verifies my theory. She lost an average of 27.4 fps/inch. Which works out to just under 1% of the starting velocity. The problem is that there are lots of guys who are convinced you will lose 50-75 fps/ inch of barrel reduction.
 
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ltc444

New member
Interesting.

When I was a kid, the belief on a 22 lr was that the powder burned up in the first 22 inches of the barrel. The last 2 inches was to ensure that the bullet was stabilized and achieved its best accuracy.

What effect does the shorter barrell have on inherent accuracy?
 
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Double Naught Spy

New member
It's not a myth. Velocity does change with barrel length. How much depends on the ammunition, caliber and gun.
Click to expand...

Right, and there comes a point when the barrel can be too long and you get reduced velocity as the barrel gets longer and longer.

What effect does the shorter barrell have on inherent accuracy?
Click to expand...

I take it that you didn't go to the link I supplied above?

Basically, what you learned was a myth, at least in regard to rifled barrels. As long as there is enough rifling for the bullet to attain a proper twist rate and as long as the crown of the barrel is properly cut, barrel length won't affect accuracy.

That isn't to say that a reduced velocity bullet won't destabilize somewhat at a distance closer to the rifle when it drops down from super sonic to sub sonic. That isn't an issue of barrel length, but of flight characteristics.
 
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SIGSHR

New member
In one of my copies of the Speer manual they discuss factors such as barrel wear, maching tolerances, etc., that result in shooters getting velocities that vary greatly from published lab figures.
 
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TriumphGuy

New member
It's all about physics. The perfect length barrel is one that is long enough that your powder finishes burning just as the bullet leaves it. If the powder burns before the projectile exits, friction slows it down. If the powder doesn't completely burn, anything left over is wasted.

Practicality dictates that you find a barrel length somewhere in the middle.
 
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lefteye

New member
I am not a physicist, but as a matter of physics, velocity is affected by barrel length and several other factors including, but not limited to, cartridge powder capacity, powder charge weight, powder burn rate, bullet weight, bullet diameter, bullet shape, bore rifling, bore diameter, primer, primer depth, etc. Many of these effects cannot be accurately identified and measured by the vast majority of firearms owners/shooters.

Thank you TriumphGuy - I was busy typing GPE style.
 
A

Aguila Blanca

Staff
jmr40 said:
Her data verifies my theory. She lost an average of 27.4 fps/inch. Which works out to just under 1% of the starting velocity. The problem is that there are lots of guys who are convinced you will lose 50-75 fps/ inch of barrel reduction.
Click to expand...
A lot depends on the powder, as noted early on in this discussion.

I did a similar test using four Para-Ordnance 1911s, with barrel lengths of 5", 4-1/4", 3-1/2" and 3". I fired multiple ammo types through each pistol over a chronograph and charted the results. The first problem (as others have found) was that with the chrony set 10 feet from the muzzle, once I got to shorter barrels I generated errors because too much burning detritus was being shot through the screens. So I had to move the chrony farther out.

With that rectified, I found that different ammos lost differing percentages of velocity as the barrel length decreased. The results were clear -- shorter barrels in general result in lower velocities, but the decline is not linear and the powder selection does make a difference. I can only imagine that if I could see the difference in barrels ranging from 3" to 5" in length, it must be more pronounced with rifle-length tubes.
 
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ScottRiqui

New member
I am not a physicist, but as a matter of physics, velocity is affected by barrel length and several other factors including, but not limited to, cartridge powder capacity, powder charge weight, powder burn rate, bullet weight, bullet diameter, bullet shape, bore rifling, bore diameter, primer, primer depth, etc. Many of these effects cannot be accurately identified and measured by the vast majority of firearms owners/shooters.
Click to expand...

All of those things play a part, but in the end, the only factors affecting muzzle velocity are the pressure inside the barrel, the cross-sectional area of the bullet, the mass of the bullet, and the friction between the bullet and the barrel.

The pressure inside the barrel is in units of force per area. Multiply that by the cross-sectional area of the bullet and you get the forward force acting on the bullet. The friction between the bullet and the barrel is a force that acts to slow the forward motion. Subtract the friction force from the forward force, and the net forward force accelerates the bullet (f=ma).

In a nutshell, increasing the barrel length will give you a greater muzzle velocity until you reach a point where the decreasing pressure inside the barrel does not give enough forward force to overcome the rearward force from friction. At that point, the bullet stops accelerating and begins slowing down.
 
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RaySendero

New member
Well...The only barrel I cut back from 26" to 23" where I had chronoed both before and after velocities lost 23 FPSPI. That's pretty close to the 25 myth!
 
4

44 AMP

Staff
Quote:
26 2826
24 2722
22 2713
20 2677
18 2654
16 2552

__________________

Her data verifies my theory. She lost an average of 27.4 fps/inch. Which works out to just under 1% of the starting velocity
Click to expand...

I think you are missing something. Note that there was 100fps lost between 26" and 24", but only 9fps lost between 24" and 22". 36fps between 22" and 20", and 23fps lost between 20" and 18". But again, a large loss, 102fps, between 18" and 16".

Velocity loss from shortening the barrel is not a linear constant. As this example clearly shows. With this load, certain reductions were much more significant than others. And that's the way it is with other calibers and loads.

Which reductions result in the greatest loss, and which result in minimal loss varies with a number of factors, and can be different for each different combination of factors. As a general rule its fine to say there will be velocity loss, but trying to boil it down to an average and apply that to everything is, I believe, the wrong way to go about it.
 
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jmr40

New member
The 100 fps drop between 26" and 24" seems odd. All the other velocity losses are right where I'd expect them to be. Most guns have "sweet spot where velocity gains sort of peak. With a 308 that is usually somewhere between 20-24". Normlly going shorter results in larger and larger velocity loses as the barrel gets shorter. Going longer usually results in smaller and smaller gains as the barrel gets longer.

If you look at enough of these tests you see a pattern. The .5%-1% theory has shown to be pretty accurate with almost every test I've looked at with guns with barrels between 20-26". Other than the one cut between 26" and 24" it is pretty close here, as well as in the average velocity loss.

Except for the difference between 24" and 26" this is exactly what I've seen in many, many other similar tests. Perhaps it was a fluke, or an incorrect chronograph reading, which is what I suspect.

In fact, after looking at Remingtons website I'm not so sure about any of Kraigwy's numbers. They show a 180 gr corelokt bullet getting only 2620 fps from 308 in a 24" barrel. He is showing more than that from an 18" barrel. Those numbers seem more accurate for a 30-06 than a 308.

http://remington.com/pages/news-and-resources/ballistics.aspx
 
4

44 AMP

Staff
In fact, after looking at Remingtons website I'm not so sure about any of Kraigwy's numbers. They show a 180 gr corelokt bullet getting only 2620 fps from 308 in a 24" barrel. He is showing more than that from an 18" barrel. Those numbers seem more accurate for a 30-06 than a 308.
Click to expand...

I don't see an issue there. First off, forget factory numbers, they aren't rules, and sometimes aren't even good guidelines.

Every gun/ammo combination has a unique mix of factors. Some guns shoot some ammo faster than others in the same caliber, even using the same ammo. Now, while most fall within the normal average ranges, there are those that fall outside average, on both ends. I have seen 100fps difference in three guns of the same barrel length and caliber, shooting the same ammo. its not common, but it happens.

If you get a gun that clocks 100fps faster than factory, cherish it. If you've got one that clocks 100fps slower, don't sweat it, just shoot it to its trajectory and enjoy it. Odds are it will still work for any job you would choose just fine.
 
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