Rifled, threaded barrel extension to improve muzzle velocity?

[Thermactor]

Is it possible to develop a barrel extension that would improve muzzle velocity? Let's say that it is rifled with the same twist as the barrel it's threaded to, and indexed with special thickness washers so that the grooves line up with the grooves at the end of the barrel. On the muzzle end, let's say that it's crowned as well.

If a 5.5" extension of this nature was threaded on to an M4-length barrel, I'm sure that the 20" overall length won't prove as fast as an ordinary full 20" barrel, but maybe it could bring velocities up from the 14.5" barrel standard up to a 16" or 18" barrel?

Just curious. I've been wondering about this for a while.

 

[Arizona_Mike]

Well you get a boost from a silencer and that is not even close to bore diameter. A grove diameter extension may give velocity. A rifled extension would have to be timed exactly with the rifling twist of the barrel or it would result in an obstruction. Even if small it would probably damage bullets and harm accuracy significantly.

Mike

 

[rs7]

Just buy another upper. It'll be cheaper, stronger, and more accurate.

 

[WestKentucky]

I have seen shotgun barrel extensions for this purpose. I would look at smooth bore extensions very slightly oversize (like the size of the bullet plus .005 so that you will not interfere with spin and you get close to maximum boost because it's still almost sealed.

 

[Arizona_Mike]

Didn't notice the question was about an M4. You are going to end up with a grossly overgassed gun doing that.

Like rs7 said, cheaper and more accurate to buy another upper. I share BCGs and charging handles between uppers.

Mike

 

[Andrew Leigh]

Well you get a boost from a silencer and that is not even close to bore diameter. A grove diameter extension may give velocity. A rifled extension would have to be timed exactly with the rifling twist of the barrel or it would result in an obstruction. Even if small it would probably damage bullets and harm accuracy significantly.

Mike

You are lucky Mike. Never had any increase in velocity with my silencer.

 

[rcmodel]

No.
It is not possible.

The insurmountable problem of matching the bore dia. & exact rifling groove dia. & twist with a threaded extension is beyond possibility.

Then if they did, they wouldn't after the treads wore in from removal and retightening a few times.

Were it possible, and feasable, 16 or 25 companys would already be selling 16" rifles with 2", 4", 6" 8", 10", & 12" extensions to take it all the waly out to 28" or more.

On top of that, what a threaded joint in the barrel would do to barrel harmonics & accuracy would be a pozzlement.

And as mentioned, the gas port location & size would change too.

Never been done, never will.

rc

 

[Thermactor]

Well it wasn't meant to be specifically about a gas-operated rifle, what about for a pistol then? And instead of rifling it, what if it's just smooth inside?

 

[Arizona_Mike]

You are lucky Mike. Never had any increase in velocity with my silencer.

It's often called "boost" and when noticeable is usually a bad thing (making ammo expected to be subsonic transonic).

It is more pronounced with shorter barrels but measurable even with quite long barrels:
http://precisionrifleblog.com/2013/07/16/6-5-creedmoor-barrel-length-and-muzzle-velocity/

In the linked case even a very "loose" .30 cal suppressor on a 6.5mm gave about 2.5 inches of barrel worth of velocity.

Mike

 

[briansmithwins]

You could, conceivably, do a squeezebore like the Littlejohn adaptor the Brits used in WWII.

Much cheaper to just replace the barrel.

BSW

 

[wally]

You are lucky Mike. Never had any increase in velocity with my silencer.

Actually unlucky, especially when the inexpensive .22lr ammo gets just enough boost to be supersonic and hence a lot louder than you'd like.

 

[BCRider]

I can see only issues with such an extension.

First off a thread on/thread off extension is never going to line up perfectly and smoothly. Threads are threads so there is always going to be SOME tolerance in the axial alignment unless the bore is drilled and rifled in the same holding setup as the end threads are cut.

Then there's the issue of matching the rifling lands and grooves. Not only do you want the lands to line up but each extension would need to have a matching land and groove sizing and pattern and twist rate. We're talking about a lot of variables here. And variables that need to be executed to a very small tolerance. Ideally such extensions would be drilled and rifled in the same setup then the ends cut off the barrel blank at the factory and matched up to each root barrel. Matching serial numbers would be a good idea.

A smooth bore extension is simply not going to work. First off the smooth bore friction will slow down the bullet spin. Or if the bore is large enough to avoid firm contact with the bullet skirt then it will not fully support the bullet. Then you'll get gas blow by and bullet deflection where it begins to wobble or diverge within the bore of the extension or possibly even hit the side wall.

So all in all while it might be possible to make such a thing I suspect that it would need to be done as a "barrel set" with some manner of a specialty connection joint that is not wear sensitive and which ensures a more stable and wear tolerant line up than a simple thread on fitting.

 

[Andrew Leigh]

It's often called "boost" and when noticeable is usually a bad thing (making ammo expected to be subsonic transonic).

It is more pronounced with shorter barrels but measurable even with quite long barrels:
http://precisionrifleblog.com/2013/07/16/6-5-creedmoor-barrel-length-and-muzzle-velocity/

In the linked case even a very "loose" .30 cal suppressor on a 6.5mm gave about 2.5 inches of barrel worth of velocity.

Mike

Thanks Mike for that and the link.

My only silenced rifle is a .375 throwing 300gr. lead at 2 500fps. I would probably on see about 20fps which equates to 0.8% to technically there is an increase but practically it is probably not material. I thought you might be talking about 100fps plus.

Either way interesting thanks.

 

[Thermactor]

BCRider
Thanks for the explanation
You buttoned this up pretty well.

Although... for a smoothbore like a shotgun i guess it should work fine then, right?

 

[BCRider]

You're welcome. Mind you I'm hardly the Higher Authority on such a topic. I just know enough about metal working and the machining processes to know that threaded connections for such an application have too many issues. It would take some head scratching to come up with a suitable stable and repeatable connection which won't end up being overly bulky.

For shotguns? NO PROBLEMO! ! ! ! After all Mossberg and a few others have made the end mounted Polychoke and similar units for just this sort of use. And, of course, LOTS of shotguns already pretty much have this feature in the form of the screw in optional chokes. But then we aren't spinning the shot cup and we're not looking for fractions of a minute for accuracy.

 

[taliv]

i don't think it is all that difficult to add the extension. i think accuracy would probably suffer a bit. i don't think any of the problems are insurmountable though.

but at the same time, i don't see it as all that useful either.

 

[Macchina]

If you really wanted to do this, couldn't a manufacturer take a 20" (or probably a 20.25") barrel and chop off the first X inches. The CNC lathe that separated the sections would have a tailpiece that held the separated section and precisely machined both faces of the cut (let's say take out exactly 0.25"). Both pieces would then be index machined (possibly a slight V and mating surface cut into each face) with a free-floating nut that would attach the two pieces together, properly indexed and never to wear or move out-of-time...

 

[Vern Humphrey]

The Germans' Paris Gun of WWI (sometimes miscalled, "Big Bertha") had a barrel too long for any of the Krupp rifling lathes. The extra length was needed to reach Paris, so they fitted it with a smoothbore extension -- and it worked just fine.

 

[denton]

It's very hard to make a truly rigid threaded joint. That's one technical hurdle right away.

 

[RetiredUSNChief]

Properly designed, a barrel extention can radically increase velocity and power. Here's a definative video on the subject:

http://www.youtube.com/watch?v=bRYRLSYVhfo

 

[Jim Watson]

If you really wanted to do this, couldn't a manufacturer take a 20" (or probably a 20.25") barrel and chop off the first X inches. The CNC lathe that separated the sections would have a tailpiece that held the separated section and precisely machined both faces of the cut (let's say take out exactly 0.25"). Both pieces would then be index machined (possibly a slight V and mating surface cut into each face) with a free-floating nut that would attach the two pieces together, properly indexed and never to wear or move out-of-time...

Well, what the old time gunsmiths building benchrest muzzleloading "slug" guns did was to drill and ream the barrel blank, cut off a piece to use for a false muzzle for precise loading, install alignment pins, then clamp it back in place and rifle all together for perfect alignment.

But I think it would be a lot simpler to just shoot the longer barrel in the first place.