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  #1  
Old 11-11-2004, 01:08 PM
Chairman Pow Chairman Pow is offline
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Does the length of a gun barrel have any effect on the power(?) of a bullet?

I understand that a longer barrel will increase accuracy (to a degree, I'm sure there's a practical limit), but does having a longer barrel increase the power of a bullet?

I'm probably not using the currect terminology here, by power, I would mean: stopping power, penetration, muzzle velocity, tumbling?

Perhaps a better question is "does a longer barrel affect any of the things I mentioned above.

As you can tell, I don't know much about guns.
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  #2  
Old 11-11-2004, 01:15 PM
js_africanus js_africanus is offline
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IIRC, a longer barrel allows more time to accelerate the bullet which translates into more speed.
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  #3  
Old 11-11-2004, 01:17 PM
Blackclaw Blackclaw is offline
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A longer barrel will produce a higher muzzle velocity.
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  #4  
Old 11-11-2004, 01:21 PM
Squink Squink is offline
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Here's a table of the effect of barrel length on muzzle velocity.
For a centerfire rifle with a muzzlevelocity in the range of 2500-3000 feet per second, each inch you shorten the barrel by will decrease that velocity by about 20 feet per second. The decrease in velocity probably goes nonlinear as the barrel gets shorter.
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  #5  
Old 11-11-2004, 01:46 PM
Mk VII Mk VII is offline
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once you've burnt all the powder in the cartridge case behind the bullet as it accelerates up the bore, additional barrel length will add nothing (conversely, if you cut barrel length to the point where you are burning part of the charge outside the barrel you are wasting powder). Long, heavy barrels will also tend to droop under their own weight (unless supported at the muzzle) adding their own problems. This last is more of a problem with artillery barrels
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  #6  
Old 11-11-2004, 02:42 PM
zoid zoid is offline
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Quote:
Originally Posted by Mk VII
once you've burnt all the powder in the cartridge case behind the bullet as it accelerates up the bore, additional barrel length will add nothing (conversely, if you cut barrel length to the point where you are burning part of the charge outside the barrel you are wasting powder). Long, heavy barrels will also tend to droop under their own weight (unless supported at the muzzle) adding their own problems. This last is more of a problem with artillery barrels

I have a question about this. It seems to me, that as long as there is more gas pressure behind the bullet than there is drag from contact with the barrel, the bullet will continue to accelerate. If this is the case, even after the propellant is all burned, if significant pressure has built up behind the bullet, won't it continue to accelerate as it travels down the barrel?
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  #7  
Old 11-11-2004, 02:52 PM
Padeye Padeye is offline
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Quote:
Originally Posted by Chairman Pow
I understand that a longer barrel will increase accuracy (to a degree, I'm sure there's a practical limit), but does having a longer barrel increase the power of a bullet?
This isn't always true. Long barrels are often used on target rifles because greater weight dampens shooter movement and to get a long sight radius (the distance between front and rear sights) as this aids accurate aiming. Some benchrest shooters use relatively short but extremely thick barrels so any vibration in the weapon will have less time to displace the shot. Benchrest is shot with optical scopes so sight radius is not a factor. Some conventional target rifles get the best of both worlds by using a "bloop tube," an artificial barrel extension. It puts more mass at the muzzle for steadiness and makes for a longer sight radius.

Virtually no centerfire small arm has a barrel long enough to give a cartridge's maxiumum potentnial velocity. It doesn't matter if all the propellant is burned before the bullet leaves the muzzle because the hot gas is still expanding and pushing the bullet faster as long as the pressure against the base is greater than friction in the barrel. .22 rimfire is different and most things I've read put optimal barrel length for .22 LR at around 18".

I don't have the magazine anymore but someone did a test with .32 special ammunition and long pieces of barrel liner placed end to end. One of the goals of the test was to see if they could make a barrel so long that the bullet just fell out the end of the muzzle. They never accomplished that because the bullet would jsut stick at any velocity below about 200 feet per second.
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  #8  
Old 11-11-2004, 03:51 PM
Crafter_Man Crafter_Man is offline
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Padeye is right on, as usual.

It really comes down to this:

If a barrel is too short, the muzzle velocity will be low. This is because the gas doesn’t spend much time pushing/accelerating the bullet.

If a barrel is too long, friction between the bullet and barrel will rob kinetic energy from the bullet, thus the muzzle velocity will be low.

What this means, of course, is that there’s an “optimal” barrel length for each caliber wherein maximum muzzle velocity can be achieved. But as many have said, most barrels are shorter than this optimum length for practicality reasons.
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  #9  
Old 11-11-2004, 07:02 PM
alterego alterego is offline
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Quote:
Originally Posted by Crafter_Man
Padeye is right on, as usual.

It really comes down to this:

If a barrel is too short, the muzzle velocity will be low. This is because the gas doesn’t spend much time pushing/accelerating the bullet.

If a barrel is too long, friction between the bullet and barrel will rob kinetic energy from the bullet, thus the muzzle velocity will be low.

What this means, of course, is that there’s an “optimal” barrel length for each caliber wherein maximum muzzle velocity can be achieved. But as many have said, most barrels are shorter than this optimum length for practicality reasons.
But what is this optimal length for say a 50 caliber machine gun? I find that more interesting than the chart in the linked page.

Would the bullet travel farther with with no barrel, or with a barrel perfectly crafted to extend for the entire range, including drop?
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  #10  
Old 11-11-2004, 07:04 PM
alterego alterego is offline
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While I'm at it, what are the effects of firing a gun in zero gravity?
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  #11  
Old 11-11-2004, 07:46 PM
Padeye Padeye is offline
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This has been covered several times already. It has virtually no effect on interior ballistics - what happens while the bullet is in the barrel. Outside the barrel, presuming there is also no atmosphere to interact with the spinning projectile, the bullet would fly in a straight line until acted upon by an outside force.
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  #12  
Old 11-11-2004, 08:24 PM
Padeye Padeye is offline
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Quote:
Originally Posted by alterego
But what is this optimal length for say a 50 caliber machine gun? I find that more interesting than the chart in the linked page.

Would the bullet travel farther with with no barrel, or with a barrel perfectly crafted to extend for the entire range, including drop?
I'm pretty sure someone could calculate the optimal barrel lengh but I'm equally sure it would be impractacally long to build.

Interior ballistics is a tricky subject but here's the 5˘ tour that might explain the no barrel vs. 1,000 yard barrel.

Contrary to popular belief modern smokeless propellant is not an explosive. I could dump out a pound of fast burning W231 and light a match to it. I'd get a big pretty FWOOSH but no boom. If I confined it that woudl be a different matter. That's how guns works. The speed of combustion of smokeless propellant is a function of the pressure it is under. No pressure and it just burns. High pressure and it burns so fast it appears to explode. True explosives such as RDX, nitroglycerine, lead styphnate, fulimate of mercury or black gun powder will explode even when not confined.

A small explosive charge, the primer, is detonated by a firing pin or an electrical impulse as in the M61 Vulcan. That causes a jet of flame to shoot into the cartridge where the powder is and ignites it. The case is partially confined by the crimp that holds the bullet in place. This moderatately raises pressure in the first few microseconds.

It doesn't take much pressure though for the bullet to start moving so it slides out of the case and makes a very short jump where there is virtually no friction at all. Then it hits the rifling. Rifling is normally described as a groove but in this case it's better to describe them as raises ridges that run the lenght of the inside of the barrel in a spiral pattern. The bullet has to swage, or squeeze itself to the shape of the rifled bore. The friction here causes pressure in the chamber to rise dramatically, speeding up combustion and pushing the bullet into the barrel.

Once the bullet is fully engaged in the rifling and moving the friction is actually less than when it first started. This is dynamic friction vs static friction. it's why a car will take more distance to stop if the brakes are locked up than if the wheels are still turning but almost ready to lock up. The bullet accelerates down the barrel, pushed by the pressure of the gas behind it. This is a very critical step. Getting any number of minute parameters wrong can cause the whole thing to explode like a bomb.

If you had no barrel there would be little energy imparted on the bullet. I wouldn't want to stand in front of it but it would be a fraction of what it shoudl be.

If the barrel was ridiculously long the bullet would continue to gain velocity until the gas pushing it didn't have enough pressure to overcome the friction of the barrel. Remember that as the bullet travels down the barrel the gas is expanding into a bigger space and cooling slightly. At a certain point the bullet would start slowing until the pressure and inertia are overcome by friction then it would just stick in the barrel.

This sometimes happens in normal barrels for various reasons. It is called a squibb load and is usually due to an insufficient or missing powder charge. The squibb is no big deal but if a shooter doesn't realize it and fires another round it can cause a gun to blow up. I've seen this happens with revolvers and semi automatic pistols and both times there was fortunately no injury. With a rifle it can easily be fatal.
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  #13  
Old 11-12-2004, 12:15 PM
FlippyFly FlippyFly is offline
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Did you recently watch the 1989 Batman movie, where the Joker pulls out a revolver with a reallllllllly long barrel and shoots down the bat-plane with it?
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  #14  
Old 11-12-2004, 12:27 PM
Padeye Padeye is offline
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Yes. The Joker's gun might be an example of a barrel longer than optimal length, especially for a revolver. Between the very low ratio of propellant mass to bore volume and with plenty of time for pressure to bleed out the cylinder gap I wouldn't be suprised if a real world revolver of those dimensions only resulted in bullets getting stuck partway down the barrel.

AFAIK the longest barrel available on a production handgun was on the interchangable barrel Dan Wesson at 15".
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  #15  
Old 11-12-2004, 02:25 PM
Quercus Quercus is offline
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One minor mistake in Padeye's otherwise excellent, thorough and accurate report.

Quote:
[dynamic friction vs static friction] is why a car will take more distance to stop if the brakes are locked up than if the wheels are still turning but almost ready to lock up.
Locked-up brakes aren't really an example of static vs dynamic friction. And in fact, you'll stop shorter with locked-up brakes -- the reason to avoid locking up is to avoid going into an out-of-control skid.

A better example of static vs dynamic friction is why it takes a bigger push to start a sled moving than it does to keep it moving -- like a sled stopped on a hillside will keep sliding if it has a tiny push to get it started.

But don't take this tiny nitpick as anything other than a 'keep up the good work", Padeye
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  #16  
Old 11-12-2004, 02:43 PM
MikeS MikeS is offline
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Quote:
Originally Posted by Quercus
And in fact, you'll stop shorter with locked-up brakes -- the reason to avoid locking up is to avoid going into an out-of-control skid.

A better example of static vs dynamic friction is why it takes a bigger push to start a sled moving than it does to keep it moving -- like a sled stopped on a hillside will keep sliding if it has a tiny push to get it started.
Nitpick of the nitpick: I don't think this is true. If you stop a car with the wheels locked, then the tire rubber is sliding across the pavement and dynamic friction is applicable. But if you stop a car with the wheels still turning, then the tire rubber is at rest with respect to the ground when it touches the pavement and thus static friction dominates. So the stopping force will be greater if you don't lock your wheels.
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  #17  
Old 11-12-2004, 02:51 PM
Crafter_Man Crafter_Man is offline
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Also keep in mind that maximum muzzle velocity does not necessarily mean the bullet will be very stable when it exits the muzzle. In fact, it could cause instability. Also, the method for achieving maximum muzzle velocity (long barrel length) has a drawback, in that the barrel’s natural resonate frequency will be low. This is why a lot of bench rest shooters are fond of short, fat barrels; they boost the barrel's natural resonate frequency.
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  #18  
Old 11-12-2004, 08:42 PM
Crafter_Man Crafter_Man is offline
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I was just thumbing through Understanding Firearm Ballistics by Robert A. Rinker. To paraphrase, it says almost all rifles have barrels that are shorter than the "optimum" length (i.e. the length that gives maximum muzzle velocity). You can see why when you view the barrel length vs. velocity data for the .30-06 (data acquired by Springfield Armory):

24" barrel -> 2710 f.p.s.
28" barrel -> 2775 f.p.s.
30" barrel -> 2830 f.p.s.
32" barrel -> 2848 f.p.s.

Much of his explanation has to do with "expansion coefficient" of the gas.

One notable exception is the .22 rimfire long-rifle bullet. According to the Mr. Rinker, years ago Eric Johnson of Hoffman Arms Co. demonstrated that an 18" barrel gave the best muzzle velocity for the .22 LR bullet.
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  #19  
Old 11-13-2004, 01:42 PM
Padeye Padeye is offline
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I don't think that .22 LR is an exception more than just an example of how scale and configuration is a big factor. The tiny powder charge in a .22 doesn't produce much gas volume to fill a long barrel. Scale it up by a factor of two and you are remarkably close to the size of a .44 mag/45 Colt. Four times the surface area for friction but potentially eight times the powder volume. The 30-06 goes even further because it's a bottleneck design with a case half again larger in diameter than needed for a .30 caliber bullet as well as being much longer.

Does anyone remember the "supergun" that Gerald Bull allegedly built for Saddam Hussein? It was so long that the barrel had to be bolted together in sections and the whole thing mounted like an elevated train going up a mountain. I wonder if there are any reliable technical specs on it. Imagine getting a bullet stuck in that barrel
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  #20  
Old 11-13-2004, 02:07 PM
Squink Squink is offline
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Quote:
Originally Posted by Padeye
Does anyone remember the "supergun" that Gerald Bull allegedly built for Saddam Hussein?
A Brief History of the HARP Project
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  #21  
Old 11-13-2004, 05:04 PM
Gunslinger Gunslinger is offline
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One of the firearms periodicals (Guns & Ammo, most likely) once had an article about the effect of barrel length on muzzle velocity. They got the longest barrel they could order, chambered for a rather low-powered cartridge (.32 ACP, IIRC), and fired an bunch of shots through a chronograph while cutting the muzzle end off in one-inch increments. The conclusion was that the longer barrel made the bullets faster, up to a point at which the propellant gases stopped expanding and friction started slowing the bullet down. Optimum was somewhere around 30 inches, I think.

The optimum barrel length for a .50 BMG is really long, because it uses relatively slow-burning powder.


There's a theory that shorter barrels are more accurate, because they're stiffer and less likely to twist a tiny bit due to the inertial forces of the rifling. This is more prevalent among users of slow cartidges, like the .45-70, that wouldn't gain much from a longer barrel.
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