Muzzle velocity question

[Don Vito Corleone]

Here's one for the physics experts out there; forgive me for the very elementary (and likely dopey) nature of the question.

It is common knowledge that the longer a gun's barrel is, the higher its muzzle velocity, assuming equal weight, equally-loaded rounds. Why is this?

My crude layperson's assumption has been that the explosion of the primer burns off the air in the barrel (or at least its oxygen), creating a vacuum through which the round travels more easily than in the ambient air. Once the bullet emerges from the barrel, wind resistance comes into effect, causing it to lose velocity as it travels. Is this even close?

Please be gentle, as I am the farthest thing from a physicist (my undergrad degree was a BA, and I barely made it through high school physics).

 

[ThePresence]

haha. Someone named Don Vito asking a gun question.
Hint to everyone else: You gotta answer with respect!

 

[Howard]

There's a vacuum in the barrel? Far from it...

 

[Viper GTS]

The longer you push something the more energy you can give it.

Viper GTS

 

[Zebo]

Becasue of rifleing. The bullet spins more because it travels a further distance down a longer rifleld barrel. This gives the bullet a straighter flight due to angular momentum and is less affected by wind resistance than a bullit from a short barrel.

 

[Don Vito Corleone]

<< haha. Someone named Don Vito asking a gun question.
Hint to everyone else: You gotta answer with respect! >>



You honor me and my family with your respect, but please call me Godfather.

 

[StageLeft]

<< Becasue of rifleing. The bullet spins more because it travels a further distance down a longer rifleld barrel. This gives the bullet a straighter flight due to angular momentum and is less affected by wind resistance than a bullit from a short barrel. >>

Well that won't increase it's velocity though; just aid it's travel.

I think viper is on to something. I'd guess that on a shorter barrel the explosion of the gun powder has less time to push the bullet because as soon as the bullet leaves a 6 inch barrel the gas then all dissipates to the side but on an 18 inch barrel the gas has only one direction to go for a longer period of time so it gives more time for the bullet to be accelerated.

 

[Don Vito Corleone]

<< Becasue of rifleing. The bullet spins more because it travels a further distance down a longer rifleld barrel. This gives the bullet a straighter flight due to angular momentum and is less affected by wind resistance than a bullit from a short barrel. >>



Hmmm - I had not considered that. So essentially a longer exposure to the rifling increases the rate of spin, and thus decreases the wind resistance?

 

[ViperMagic]

But its effect becomes nonexistant once your barrel is so long that the gas is all expanded down the whole tube

 

[ScottMac]

The slug only accelerates while the powder is burning. Once the powder has completely burned, the bullet begins to slow down, moreso if it's still in the barrel (friction from the barrel). If all of the powder has burned, and the bullet is only halfway down the barrel, you've lost some velocity by the time the bullet has left the barrel.

Longer barrels are desirable because they offer a longer sight radius (the distance between the front and rear sights). Shorter radius equates to more error amplification when aiming.

The ideal setup is a powder mix that burns until the bullet is about 90% down the barrel, with a barrel length of sufficient length to offer reasonable accuracy for the range-to-target. You do not want the powder burning to the very end of the barrel or beyond, because the additional push/fireball/whatever tends to de-stabilize the bullet (already destabilized somewhat by the transition from the barrel to the open air).

This is a very light scratching-of-the-surface / simplifcation, there are a mess of other factors regarding the powder, the twist ratio of the lands & grooves, bullet shape, etc.....but perhaps it'll give you an idea of the concept.


FWIW

Scott

 

[notfred]

Gunpowder explodes: Hot expanding gasses push bullet. When the bullet reaches the end of the barrel, the gasses are no longer trapped behind it, forcing it forward, and the bullet stops accelerating. The longer the barrel, the longer the bullet gets pushed by hot gasses. Of course, if you're barrel was long enough so that the bullet got to the point where the gasses behind actually stopped expanding and started to cool, then the bullet would slow down anyway. That would be a pretty long barrel, though.

 

[Zebo]

It is common knowledge that the longer a gun's barrel is, the higher its muzzle velocity, assuming equal weight, equally-loaded rounds. Why is this?

I thought you asked about travel earlier...

Rifle rounds such as 30-06, 7mm, 308 etc have much more powder than a short muzzle gun rounds like a .357. 44mag etc...This is why there faster and require a heavy duty breach to channel the explosion without destoying the gun.

Actually, I think the muzzle exit velocity would be faster if you cut a rifle barrel in half (assuming the same round) because there would be less friction caused by the shorter barrel. But that bullet would not travel as far as if you left it in tact, becasue the velocity would quickly slow due to wind resistance.

 

[tcsenter]

As I understand it, please correct me if I'm wrong:

Longer barrels allow the expanding gases of combusting/burning powder to act upon the bullet for a longer interval of time, increasing the potential to impart more energy and velocity to the bullet. Acting within this dynamic, are different burn rates for powder: slow, medium, fast, etc. Those are obviously not technically correct descriptions of the burn rates, just relatively speaking.

It is ideal to use slower burning powders in long barreled firearms, since the gases of combustion need not reach their expanding potential before blow-by at the end of the muzzle nearly as fast as it would need to in a handgun. There is almost no powder that can actually burn as quickly as would be ideal in a handgun, at least nothing that is safe to use in a handgun, which is why you get a pretty good muzzle flash on anything with 4" or shorter barrel no matter what ammunition you use.

Rifling has little to do with the issue of velocity, except that more aggressive twists, or the longer the bullet must travel down a rifled barrel, pose greater degrees of resistance and thus act as a parasitic influence. Rifling, though, is primarily about stabilization during flight.

 

[Bulk Beef]

Viper is right.

 

[tcsenter]

<< Hmmm - I had not considered that. So essentially a longer exposure to the rifling increases the rate of spin, and thus decreases the wind resistance? >>

Not at the muzzle. Rifling is for down-range stability, and that stability comes at a price of increase resistance while the bullet travels down the barrel. We are talking about muzzle velocity, right?

Rifling DECREASES effective muzzle velocity by a parasitic mechanism, while the spin helps preserve down-range stability, which in turn will decrease the rate at which the bullet loses velocity due to poor flight characteristics (better drag co-effeciency). But in no way could it be said that rifling 'increases' velocity, it only helps decrease the loss of velocity.

 

[silverpig]

<<The slug only accelerates while the powder is burning. Once the powder has completely burned, the bullet begins to slow down, moreso if it's still in the barrel (friction from the barrel). If all of the powder has burned, and the bullet is only halfway down the barrel, you've lost some velocity by the time the bullet has left the barrel. >>


Wrong. There's a lot of half right answers here, and a few completely wrong ones. Here's what happens.


The powder ignites and forms a gas. This gas begins to push the bullet down the barrel. The burning of the powder to form a gas is half of what pushes the bullet down the barrel. There is now a gas behind the bullet that wasn't there before. This extra gas will exert a force (read pressure) on the bullet that is much greater than the 15 psi that is pushing the bullet towards the back of the barrel. Once the gas is formed, it continues to push the bullet along the barrel because of simple thermodynamics. A heated, pressurized gas will try to come to equilibrium with it's environment by some combination of cooling and expanding. In the case of a gun, there isn't much time for the gases to cool, so this equilibrium is attempted by expansion alone. This expansion will continue to push the bullet down the barrel until the pressure behind the bullet is equal to the pressure in front of the bullet. This point is not reached in even the longest barrelled guns because the engineers know that the bullet will begin to slow down after that point due to the pressure imbalance.

Why rifles have longer barrels and higher muzzle velocities is because the gases are allowed to expand farther down the barrel, and push the bullet for a longer time. The bullet does not slow down at any point in the barrel. Longer barrels also allow for higher accuracy and better rifling but that is just a bonus. The rifling effect does nothing to increase speed, or decrease wind resistance, but keeps the bullet truer by adding a great deal of angular momentum to the bullet. This angular momentum helps to prevent the bullet from being turned away from it's original direction, but does nothing for it's speed.