How long would the barrel have to be?

[Moparmike]

How long would the barrel have to be for the friction of the barrel overcome the force of the expanding gasses? Is there some formula that one could use to determine that? For instance, X amount of powder pushes Y weight bullet with Z legnth, with A barrel legnth and B deep barrel grooves? Could you determine a friction coefficient for copper or lead bullets on chrome or steel barrels? Can a powder's creation of gasses while burning be determined?


Wouldnt that formula work for any gun?

 

[Hand_Rifle_Guy]

I only can cite two examples.

First, a .22 LR is generally accepted to run out of expansion volume after 20" of barrel. After that, drag time.

And I read in a gunrag once about a guy and his 'smith friend who had bundle of .30-cal barrle-liner stock that was in (I think) 10' lengths. These two contrived a way to gas-tight-link these blanks end-to-end, and ran the piddliest centerfire round they could think of to experiment with, i.e. the .32 S&W. Not the Long, the original one you put in those tiny little top-break Ivers. they tried several rounds, and sdome petered out at about 18" or so, while some would go as far as about 28-29' IIRC.

.32 S&W's an old black-powder number, and I imagine it typically runs as low as 12,000-16,000 psi, much like a .38 Special. That makes me think a high-power rifle round would require a lo-o-o-ng barrel to halt a bullet. I really have no idea about how to figure the math, but someone who remembers more chemistry than I do could probably work out the amount of combustion products for a given weight of powder presuming you knew it's precise composition, which might give you an idea.

 

[BluesBear]

About 1%-2% of the time a .22 Aguila Colibri (not the Super Colibri) won't quite make it to the muzzle of a 24" barrel.

My question is,
When a barrel gets to that "magic" length where all of the power has burned and all of the gases have expanded, but right before the point that the motion of the bullet starts to create a vacuum, wouldn't the shot be silent?
If the air pressure in the barrel was identical with the outside air there should be no muzzle blast.

 

[Mulliga]

wouldn't the shot be silent?

Depends on how fast the bullet goes. As I understand it (and I'm no expert in suppressed weapons), suppressors use exactly this principle of baffles and other stuff to get the pressure of the exiting gas nearer the pressure of the atmosphere, resulting in a lot less noise. I suppose with a big enough system of baffles (i.e., sort of like your extra-long barrel), the sound would almost be entirely gone.

But don't modern bullets create most of the report of a firearms when they break the sound barrier? I've read that that's the case, and that rifle ammunition in general is much harder to suppress effectively.

 

[R.H. Lee]

I would think the friction would increase due to expansion of the bullet caused by increasing temperature as it travelled down the bore.

But then, the bullet would require less push from the expanding gases as its speed increased. However, that may not be an issue because the push of the expanding gas may so overwhelm the resting inertia of the bullet as to make it negligible. (0-1200 fps in .0000000000000001 sec.?)

I don't know what I'm talking about.

My head hurts.

 

[4v50 Gary]

For blackpowder rifles, it's about 30-32" for optimal results. Longer doesn't make it any more accurate (as related to be from his tests by Colonial Williamsburg Master Gunsmith Wallace Gusler). The real advantage of the American long rifle was the longer sight radius.