Barrel extension to improve performance?

[jkpq45]

Will a smooth barrel extension on a rifled barrel increase velocity?

For example, my .243 barrel is 22" long. If I were to thread the end and install a tube with ID equaling groove diameter, am I going to increase velocity of the projectile?

Will accuracy decrease?

Thanks!
jkpq45

 

[Sam1911]

Yes, velocity would increase (to a point) because of the pressure of the burning powder gasses having longer to act on the bullet.

This happens to a degree even in imperfect/perforated extension tubes, like suppressors. It's called "silencer boost" and can push a bullet that normally is subsonic just over the sound barrier when the can is screwed on, leading to that unexpected "CRACK!"

Accuracy is likely to suffer, but I don't know how much.

This seems like one of those things that someone, somewhere must have tried at some point.

 

[denton]

It's probably not a winning strategy.

There is no telling what it will do to accuracy. The most likely outcome is serious degradation.

You'll pick up a bit of MV, but not enough to write home about.

 

[Ian]

As a practical matter, you can't do this because you can't time the rifling accurately enough that the original barrel will match up with your new extension.

If you want a longer barrel (which will increase velocity slightly; probably not noticeably), you have to buy a new barrel.

 

[jkpq45]

I suppose I should mention, though this contraption is currently chambered in .243 Winchester, it is destined to become something that will benefit from a longer barrel.

Ian--point well taken! I don't plan on rifling the extension, just reaming it to groove diameter. Presumably, the bullet will have been engraved a certain amount by the barrel's rifling--any may have obturated close to groove diameter. Perhaps the extension can be fit closely enough to groove diameter to give a velocity boost.

Sam1911--sounds like a worthy experiment. What's a foot-long piece of 12L14 and an hour on the lathe worth, anyway? Depends on how many coffee breaks I take.

 

[Sam1911]

I'll also mention that back when I was shooting 3-position smallbore, most of the competitors used "bloop tubes" which were hollow extensions that bolted onto the muzzle and extended the gun's sight radius 6" or so.

(They also tend to make the shots a little quieter, but don't tell the BATFE that! )

I imagine that you could make a kind of a bloop tube that was nearly groove diameter but over by a few thousandths such that the bullet never would touch the tube, and that you would pick up a measurable velocity increase.

Whether an 8" or 10" tube might get you another 100 fps? I'm not sure. Probably. Would it degrade accuracy? I don't think so as long as the muzzle of the tube was well crowned and the bullet never actually touched it.

 

[Ian]

I think that if a smoothbore extension is tight enough to give you a velocity increase, it will also significantly reduce accuracy by dragging unevenly on the bullet. If it's wide enough to not touch the bullet, it probably won't increase velocity (and gas expanding past the nose of the bullet may also detract from accuracy).

An interesting reference for these sorts of questions is F.W. Mann's "The Bullet's Flight From Powder to Target". Mann did a lot of interesting experiments with rifling, among other things.

 

[Sam1911]

I think that if a smoothbore extension is tight enough to give you a velocity increase, it will also significantly reduce accuracy by dragging unevenly on the bullet. If it's wide enough to not touch the bullet, it probably won't increase velocity (and gas expanding past the nose of the bullet may also detract from accuracy).

I tend to worry that this might be the case as well, however, it is pretty clearly established that suppressors do "boost" velocities noticably, and a lot of guys doing very precise long-range shooting are using suppressors with no accuracy loss.

Wonder if Zak Smith might stop by and speak to that?

 

[pseudonymity]

My guess is that any velocity gains would be more than offset by accuracy loss. I just do not see a smooth bore being fitted closely enough to blow gas past the bullet in a stable pattern. I think you would end up creating one long, threaded bad crown on the rifle.

If you have the short action and want barrel length and velocity, you could just fit up a 32" Palma barrel - although I would not want to lug that around if this is a hunting rifle.

 

[jkpq45]

This'll be a target rifle, pseudo. If I go p-doggin', it'll be off the back of a truck.

Thanks, all for the considerations and thoughts. I'll keep noodlin' the idea around myself.

 

[colonelhogan44]

Do it and let us all know if it works!

 

[Zak Smith]

Hmm, interesting question.

Suppressors and muzzle extension tubes (for increased sight radius) can be used with no degradation in accuracy.

Baffle design in suppressors do not harm accuracy usually involves some consideration for that very purpose. It's also not uncommon for a "near miss baffle [would-be] strike" to cause an accuracy problem.

Based on this, my guess is that for a given cartridge/load there is a minimum ID that won't harm accuracy. I have a hard time believing that a close fit would not harm accuracy based on gas flow

 

[jkpq45]

Fair enough, colonelhogan44. I'm threading the muzzle for a brake anyway--may as well build this extension and see what happens.

If it doesn't work, I'll cut it down and port the silly thing and call it a brake.

 

[Sam1911]

Thanks Zak! Can you share a guess as to how much over bore diameter a tube like this (or a suppressor's baffle passage) would need to be to not cause gas pressure effects on the bullet? Are we talking about going 0.1" over? 0.05"?

 

[adelbridge]

after all is said and done it would certainly cost more than a new longer barrel even if you had the skills to do it yourself. Guns have been around a long time and if it was an effective and useful design it would have been marketed.

 

[Vern Humphrey]

For a barrel extension to improve accuracy, it has to be groove diameter -- if it's much larger, you'll just get gas blow-by and little or no increase in velocity.

Interesingly enough, the Paris Gun of WWI (often incorrectly called "Big Bertha") had a smoothbore extension. Krupp, who made the gun, did not have rifling lathes long enough for the barrel, so the get the desired length, they added an unrifled section.

 

[jkpq45]

Vern--that's what I'm thinking as well. There will still be blowby wherever the projectile hasn't obturated to full-groove diameter, but it'll at least be better than an extension at land diameter (i.e. bomb!)

 

[Zak Smith]

The aperture diameter of suppressor baffles might be only 0.060" larger (plus or minus), but the baffles are designed to strip the gas away, which a small tube would not do.

 

[Sam1911]

Ahh, that makes sense.