Barrle Length...

[The Surgeon]

I have been following another thread hear and a question came to mind. This may be irrelevant but it does interest me.

We all know that a longer barrel enables the shooter to squeeze every bit from the powder in the cartridge, right. Is it possible to have the barrel to long that one begins to go backwards? That is, can the barrel be so long that the powder/charge no longer helps the projectile or even begins to hinder the performance?


If this does occur, at what point and how would one determine it on a specific caliber/charge?

In theory there should be no effect, since the gasses are trapped. The pressure is what it is, until released. But, when does the length of the barrel, if at all, become overkill?

Of course there is another theory, inaccurate as it may be, it is still theory.
If the charge is being completely used and has applied it's maximum effect on the projectile, it would be reasonable to think that the longer the barrel is, past its beneficial length, the more resistance the charge/projectile would have to overcome this hindering the performance.
What is reality?

[shortgrass]

Originally Posted by The Surgeon

In theory there should be no effect, since the gasses are trapped. The pressure is what it is, until released. But, when does the length of the barrel, if at all, become overkill?

Of course there is another theory, inaccurate as it may be, it is still theory.
If the charge is being completely used and has applied it's maximum effect on the projectile, it would be reasonable to think that the longer the barrel is, past its beneficial length, the more resistance the charge/projectile would have to overcome this hindering the performance.
What is reality?

After all the propellant has been burned and it has reached its peak in a barrel of optimum length would not a longer barrel (more volume) have a lower pressure at the muzzle? At some point the gas quits expanding because there's no more propellant to burn. It's not like a hydrolic cylinder where you have a pump to maintain pressure throughout the stroke of the piston (bullet).

[dmontana]

Dan liljas article on barrel length
Lilja Precision Rifle Barrels - Articles: Barrel Lenghts and Velocities in the 338/378 Weatherby Magnum

[TikkaShooters]

Originally Posted by shortgrass

After all the propellant has been burned and it has reached its peak in a barrel of optimum length would not a longer barrel (more volume) have a lower pressure at the muzzle? At some point the gas quits expanding because there's no more propellant to burn. It's not like a hydrolic cylinder where you have a pump to maintain pressure throughout the stroke of the piston (bullet).

This has been my thought as well. Unfortunately I've never been able to test it.

I can't wait to see some of the answers.

[Mikecr]

Well there is diminished returns pretty quickly. And more than just w/regard to velocity.
But even with extreme barrel lengths(like 100ft), muzzle pressures would continue to drop but velocities would continue to rise. There would still be barrel pressure(~50psi)(not vaccuum).

No point here would velocity actually drop, because the bullet was engraved way back, it lost a little jacket OD and obturates less with lowering pressures, and so it's frictional resistance would remain below barrel pressure forces.
Maybe this would change with a 150ft barrel (so don't go that long)!!

[The Surgeon]

Looking at Liljas data it shows the the 250 gr projectile has an optimum barrel length of 42 & 44 inches in regards to velocity, any longer and the projectile begins to drop in velocity.

Now looking at the same data, the 300 gr is still gaining in velocity at these lengths. This is very interesting, because the only change in variables is the weight of the projectile.

I would expect just the opposite in velocities vs the 250 and the 300 gr bullets. But according to the data provided, it is not the case. By that I mean the 300 is still climbing in velocity at 46 inches where as the 250 is falling off. Why?

According to Lilja the increase in performance for the 300 was based on a heavier projectile, slow burn powder, and a longer barrel will equal an increase in overall velocity. Looking at the data on the 320 gr this theory does not apply.

[Mikecr]

I don't know what data you're referring to, but I suspect you misunderstood it.
The bullets would not drop off in VELOCITY, but instead would drop off in VELOCITY GAIN.

That is, beyond a certain point velocity gain rate decreases with increments of increased barrel lengths. But velocity IS still going up.
You want to have enough barrel that muzzle pressures are not a problem, and yet there is little gain in going more than needed for good accuracy.