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View Full Version : Longer barrel = higher velocity right?


Booshanky
12-12-2008, 10:49 AM
At what length does a barrel stop increasing the velocity of the bullet? For the sake of argument, lets say it's standard 5.56 ammo.

Frijolito1988
12-12-2008, 11:15 AM
Good question, i belive i heard or read sometime ago that for every inch a standard barrels gains or looses , its a 20fps more? Not sure , let me try and research

TurboFall
12-12-2008, 11:20 AM
http://www.ar15.com/forums/topic.html?b=2&f=134&t=149575

Data posted by MSTN on arfcom.

Booshanky
12-12-2008, 11:33 AM
There's got to be some sort of mathematical formula for it right? Like, would a 36 inch barrel result in a higher velocity or lower?

There's no real point to my question, I'm just thinking about this stuff today.

Teletiger7
12-12-2008, 11:39 AM
After a certain length(maybe really long) the velocity will start to go down due to friction/drag from the barrel. So there's some point in barrel length(I'm sure that tests have been done) that velocity will actually start to go down.

mecam
12-12-2008, 11:40 AM
36 inch for a 5.56? The bullet might get stuck on the way out. :p

AaronHorrocks
12-12-2008, 11:45 AM
It's a matter of mechanical engineering.

One more variable to consider is the powder type and load (very important).

ddimick
12-12-2008, 11:53 AM
My dad put together some numbers for me on this topic a couple days ago. He's a 50BMG shooter and has written articles for the FCSA mag, and I have a high degree of confidence he knows what he's talking about. Or at least he knows a hell of a lot more than I do, anyway. Hope this helps.

Barrel Muzzle Muzzle Muzzle Muzzle Ballistic Ballistic Dwell Dwell Powder Powder
Length Velocity Velocity Energy Energy Efficiency Efficiency Time Time Burn Burn
(inches) (feet/sec) (normalized) (ft/lb) (normalized) (%) (normalized) (ms) (normalized) (%) (normalized)
26 3182 100.00% 1236 100.00% 27.8 100.00% 1.157 100.00% 98.3 100.00%
24 3125 98.21% 1193 96.52% 26.8 96.40% 1.105 95.51% 97.8 99.49%
22 3062 96.23% 1145 92.64% 25.7 92.45% 1.051 90.84% 97.2 98.88%
20 2990 93.97% 1091 88.27% 24.5 88.13% 0.996 86.08% 96.4 98.07%
18 2907 91.36% 1032 83.50% 23.2 83.45% 0.939 81.16% 95.4 97.05%
16 2811 88.34% 965 78.07% 21.7 78.06% 0.881 76.15% 94 95.63%

PIRATE14
12-12-2008, 11:58 AM
Well....pretty sure the data is around....but as soon as the powder or accelerant stops burning/pushing....that's it.....

As per the chart above....26" sounds close....of course it's dependant on the round...

NRAhighpowershooter
12-12-2008, 1:33 PM
for poops and grins for HighPower matches.. most 'SpaceGuns' have barrels that are 26" some HighMasters have been playing around with 30" barrels with a 6" bloop tube. To date I haven't heard anything positive or negative on these long barrels

bohoki
12-12-2008, 1:42 PM
it depends on the particular load so the question is unanswerable

i too have often wondered wehre the point of diminishing returns is

i know that 24" is too much for 22 colibri

asheron2
12-12-2008, 2:09 PM
Me and a friend have been playing around with a longer barrel and different loads. The real magic for long range shooting is that by using a longer barrrel, you can use a slower, and hopefully more consistent, burning powder. This will hopefully then give you a better consistency on the velocity of the bullet when it leaves the barrel.

aplinker
12-12-2008, 4:01 PM
No boo-boo, I'm a disabled veteran in my recliner so ST*U or ESABATM?

:confused: What's with the venom?

Longer doesn't always equal faster, but you have to get pretty damned long before you stop increasing velocity - it's just you get diminishing returns and reduce accuracy, not to mention make it ungodly heavy.

I'm betting 5.56 doesn't start to lose velocity until around 36" or more.

J_Rock
12-12-2008, 4:39 PM
In general this is true but its highly dependent on the burn rates of the powder you are using. Slower burning powders means higher velocity gain from longer barrels.

EOD3
12-12-2008, 4:55 PM
:confused: What's with the venom?

Sorry, bad day, my pain medications aren't getting it done.

Fjold
12-12-2008, 5:08 PM
There are to many variables to get any exact answer.

Just off the top of my head, besides the load used, you have to consider the coefficient of friction of the barrel material and bullet jacket material. Each barrel will vary in materials used (types of steel), diameter, surface finish, inclusions and voids as will each individual bullet. The energy required to engrave the bullet and compress the core will vary and the pressure of each load will vary with different amounts of gas leakage around cartridge cases due to variations in the individual cases, differences in the gas sealing ability if different bolt to barrel fits.

pizzatorte
12-12-2008, 6:50 PM
Well....pretty sure the data is around....but as soon as the powder or accelerant stops burning/pushing....that's it.....

As per the chart above....26" sounds close....of course it's dependant on the round...

Erm, short of the chamber exploding, or all of the gas being dumped when a semi-auto cycles, the pressure behind the bullet in the barrel will continue pushing until the bullet leaves the barrel and relieves the pressure.

seanorgm@earthlink.net
12-12-2008, 7:27 PM
This is the fascinating (to me, anyway) universe of 'interior ballistics'. As noted already, there are multiple factors.

One of the primary factors is what is known as 'expansion ratio'. This is the difference between the space (volume) in the cartridge case prior to the round being fired and the total volume of the cartridge case and the barrel when the bullet leaves the barrel. Larger expansion ratios come from a. lengthening the barrel and/or b. increasing bore diameter.

However, in order to get the maximum efficiency from the expansion ratio present, one must use the proper burning rate propellant. So bigger expansion ratio firearms - straight sided cartridges like .45 ACP, .44 WCF, .32 WCF, .45-70 and .458 Winchester require faster burning powders. The other factor to determine powder burning speed is the relation of powder charge to the sectional density of the bullet. In short, the more resistance to movement, the slower the powder should be.

Obviously, .45 ACP and .458 Winchester do not use the same burning rate powder, but you'll notice the .45 ACP works best with 'pistol powders' in the middle to fast range, and .458 Winchester runs well with 'rifle powders' in the fast to middle range as well. The difference between 'pistol powder' and 'rifle powder' is not all that well defined in the smaller rifle rounds and larger pistol rounds.

Getting back to the .223 Remington - 5.56x45 mm round mentioned, the case capacity has a specific relationship (expansion ratio) to any given rifle barrel length. Comparing the 5.56 with .22-250, it should be obvious the 5.56 has a greater expansion ratio (in the same length barrel) than the .22-250. So, varmint rifles in .22-250 almost always has a much longer barrel than a 5.56 or .223 Remington.

Anyone remember "Boyle's Law" from high school physics? Internal pressure is inversely proportional to volume. So when the volume of the interior 'system' doubles, the pressure drops by half. Can't escape that, it's the law. At some point, resistance of bullet jacket rubbing on bore interior will overcome the constantly lessening interior pressure in the firearm.

So how much is too much?

I once calculated pressure drop for a 9x19 NATO round. The barrel has to be over 400 feet long in order for the interior pressure to drop to normal atmospheric pressure. Probably awkward in a holster gun or even a carbine.

In reality, the requirements of carrying and use is a far more limiting factor than losing velocity.

For specific instances, do a Google search and see if you can find an on line version of the "Powley Pressure Calculator". It was originally sold as a slide rule sort of affair, based on the mathematics I've described here in prose.

Any 'rule of thumb' unit of 'so many fps per inch' is simply an estimate at best and a guess at worst.

Have I confused you enough? Or would you like me to explain more?

There's an excellent book on the subject. Firearms Pressure Factors by Lloyd E. Brownell, Ph. D. is very good. Published by Wolfe Press in 1990. For anyone interested, it's worth having and reading.

ar15barrels
12-12-2008, 7:47 PM
Just off the top of my head, besides the load used, you have to consider the coefficient of friction of the barrel material and bullet jacket material. Each barrel will vary in materials used (types of steel), diameter, surface finish, inclusions and voids as will each individual bullet. The energy required to engrave the bullet and compress the core will vary and the pressure of each load will vary with different amounts of gas leakage around cartridge cases due to variations in the individual cases, differences in the gas sealing ability if different bolt to barrel fits.

I have heard numbers ranging from 7000PSI on small caliber rifle bullets to 1500PSI on large caliber pistol bullets as the point where dynamic bore friction overcomes acceleration generated due to pressure.
Quickload does not account for this in their output.
Here is a 48" barrel with a max load of H335 under a 55gr hornady bullet in a 223:

http://ar15barrels.com/tech/48inch223.gif

The pressure drops under 7000PSI at about 32" of barrel.
That's where my SWAG says that this load would start losing velocity.

Here is a plot for 9mm in a 10ft long barrel:

http://ar15barrels.com/tech/10ft9mm.gif

Note that the pressure is under 1000PSI already at 20"
I'll bet that a 24" 9mm barrel would show lower velocities than a 20" 9mm barrel.

ar15barrels
12-12-2008, 7:55 PM
i know that 24" is too much for 22 colibri

Yep.
They sometimes stick in my 27" barrel.
However, I only stick 1 or 2 per 50rd box.
That tells me that those 1 or 2 were different than the other 48 or 49 rounds...

ar15barrels
12-12-2008, 8:00 PM
Erm, short of the chamber exploding, or all of the gas being dumped when a semi-auto cycles, the pressure behind the bullet in the barrel will continue pushing until the bullet leaves the barrel and relieves the pressure.

True that the pressure continues to push, however, there is a point where dynamic bore friction overcomes the pressure generated by the propellant.
At this point, the acceleration becomes neutral.
As the bullet continues, the pressure will keep dropping and the bullet will start decellerating.

To actually have the bullet STOP in the bore is a another whole level...
I read about a test where a ballistician put together enough barrels (including timing of the grooves/lands) to make a 10ft long barrel.
He was sometimes able to get bullet from a 32 colt to stop within the 10ft barrel, but the bullet usually made it out of the barrel.