Hypothetically long barrels


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Thernlund
January 23, 2008, 02:54 PM
I saw this thread (http://www.thehighroad.org/showthread.php?t=333673) and did a Google search for some long barrels. Then I started thinking...

I wonder how long a barrel would have to be before the bullet would not be able to exit the end? This would certainly depend on the cartridge no question. I imagine it would also depend on the type of action. For example, a revolver has a flash gap that will allow pressure to escape faster than and auto would, etc.

So thinking about this, I think the main factors would be the bullet's friction on the barrel, the charge behind it, and how fast the gases could escape. As well, the area inside the barrel behind the bullet as it moved away from the chamber would factor.

I did some quick math. If the chamber and the bullet created perfectly sealed space between them, a .45ACP would travel 682' down a barrel of infinite length before the pressure reached below 2 psi. It would likely stop before that from friction. I used 21,000 psi as the chamber pressure and assumed that pressure would be halved as the distance from the chamber doubled (1", 2", 4", 8", etc).

Dumb thought experiments, I know. I once asked how physically long an Ethernet pack was (about 100km from a T1 as it happens).

Comments?


-T.

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.455_Hunter
January 23, 2008, 03:13 PM
I remember reading in one of the gun rags are few years ago (I think G&A) about a test using the .32 S&W cartridge and 8' sections of rifled barrel liner. There definitely was a point where the bullet just stopped in the "barrel". I am not sure exactly, but around 16' seems to ring a bell for that particular cartridge.

rcmodel
January 23, 2008, 03:21 PM
a .45ACP would travel 682' down a barrel of infinite length before the pressure reached below 2 psi.I don't think that is even close to right.

682 inches maybe, but certainly not 682 feet!
And I think even 682" (56.8 feet) would be a far stretch!

Pressure is not the determining factor.
Gas volume / expansion ratio & bullet friction would be the determining factors.
And a .45 ACP doesn't have much gas volume when it starts out with only 5.0 or 6.0 grains or so powder to burn.

PS: The powder gas would quickly cool off, condense, and loose pressure / volume in any really long barrel.

It wouldn't be just a straight pressure decrease based on increasing bore volume at a fixed starting pressure.

http://i81.photobucket.com/albums/j219/rcmodel/KTOG/1224.gif
rcmodel

DoubleTapDrew
January 23, 2008, 03:32 PM
Aren't there some rounds (primer-only .22lr IIRC) that won't even make it out a standard 10/22 barrel on occasion?

Claude Clay
January 23, 2008, 03:32 PM
didnt jules vern get to the moon once? more like a morter as i remember though.

2tapdrew....CB shorts or longs. and they exit very well from my cz 452 (28 1/4" ) into about 50 pages of phone book.

Thernlund
January 23, 2008, 03:33 PM
I don't think that is even close to right.

It assumes alot. For example, near zero friction on the bullet, and a constant pressure.

I guess what that statement is saying is that 21,000 psi at the chamber would be under 2 psi at 680+ feet given a perfect seal and constant temperature.

As you point out though, other factors will have a bigger impact. Gas temperature and bullet friction for example, not to mention the fact that gases would escape.

I'm thinking around 20' to 25' for a .45 ACP. Maybe 30' to 40' for a .308 Winchester.


-T.

JesseL
January 23, 2008, 03:39 PM
Don't forget that atmospheric pressure is around 14psi.

If we were to ignore friction and assume perfect sealing, inertia would carry the bullet down the barrel past the point where chamber pressure equaled atmospheric pressure and then the bullet would reverse direction and start oscillating around the point of equilibrium.

Zoogster
January 23, 2008, 03:46 PM
The problem with the assumption is most barrels are not smoothbores. Rifling imparts greater friction and uses up more energy the longer the bullet is subjected to it.
Rifling is unique and there is many types and even the same types have variations. That means an equation for the exact amount of friction would vary greatly.

A related note I find interesting is the point at which rounds stop gaining velocity from a barrel and begins to lose velocity. For some rounds it is around 16" (something like that for a .22LR) for others it is longer or shorter (most longer).

There is also a point where dramatic gains cease to be made, and only minor gains continue to be achieved. Those are best represented on a curved graph and good for understanding acceptable and unacceptable tradeoffs with various pistol length barrels. Obviously the barrel will be much shorter than the point it would stop gaining velocity in a pistol, however at different points the percentage gained changes dramaticly.

Another neat thing to figure out is the amount of time the projectile will be in the barrel vs the amount of time before the recoil impulse effects the firearm. If the bullet leaves the barrel prior to the recoil impulse effecting it, then recoil has no effect on accuracy. This is noticeable in some shotguns as the cutoff for many slugs is between a short barrel (18") and a long barrel. That is why slug guns are best with short barrels. That is determined by many things, including weight of the firearm, payload, and velocity of the payload. So it is unique to every firearm and worth figuring out and understanding. If the barrel is short enough to allow the projectile to escape prior to recoil then it is always as accurate as the shooter. If however the recoil impulse has an impact prior to the projectile leaving then it varies more, will have a higher point of impact, and how stiff the firearm is held will greatly determine point of impact.


However there is little purpose is determining exactly when a bullet would stop, because the barrel length is unrealisticly long, and bullet performance will have seriously degraded (started to lose velocity) at a point long before that.

Thernlund
January 23, 2008, 03:51 PM
The problem with the assumption is most barrels are not smoothbores.

It was an unrealistic calculation just to figure the pressure. No bearing on reality.

However there is little purpose is determining exactly when a bullet would stop...

Sure there is! I'm bored. :D


-T.

Chuhhuniban
January 23, 2008, 04:13 PM
Sure there is! I'm bored.


-T.

Absolutely right on! I can remember sitting in an US Army Ordnance Study Group debating how fast the cyclic rate on an M16 might get before the mechanism surrendered to inertial forces and shattered. And, in another instance, listening to a bunch of bored civil engineers discuss how far on to a sub-standard railroad trestle a GM EMD Gp-9 locomotive could get before it fell through. All such discussions are improved by beer and places to put up your feet. They are damaged by the one guy who gets up and goes to look for the book.

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