Gunsmithing Muzzle Threading ?

[TheBeard413]

I have been a difference in diameter between the bore at the chamber and muzzle. Does anyone has experience with the belling of the barrel when 5/8"x28 TPI? I checked mine with a Go- No Go gauge and my Bartlein Barrel has a slight bell in the area of the muzzle threads.

I noticed that the maximum-sized bushing that would fit in the bore at the chamber end was almost .0015” smaller [than what would fit] at the muzzle.

Has anyone found a solution?

 

[ken226]

What cartridge is it chambered in?

1/2-28 shouldnt be a problem in .223, but could be an issue on larger bores.

Did you do the same checks before the muzzle was threaded? Lapped barrel blanks tend to have slightly larger bores near the ends due to the lap changing direction during the process.

Is it also possible that whoever chambered the barrel didnt cut enough off the muzzle-end of the blank to remove the bellmouth portion?

What was the blank length, and the finish length?

 

[TheBeard413]

.300 RUM, 30" blank - 26 1/2" finish

 

[ken226]

Sounds like your gunsmith took plenty off so the lapping bellmouth isnt likely the issue.

But,

Id have serious reservations about using a 1/2-28 thread on a .30 cal, let alone a magnum.

I'll do a hoop stress calculation right quick and see if thats your problem.

 

[ken226]

Im getting about 1800psi at the muzzle. With a wall thickness of .115" (1/2-28 bore and a .461 minor diameter divided by 2) which results in a hoop stress of 3607psi.

The tensile strength of 416r being about 91,000 psi , you shouldnt even be close to its yeild. So i doubt its a pressure yield issue.

I think the 416r steel had a little compression stress and turning it so thin resulted in the bore material being relaxed a little.

At full diameter it was a consistent diameter. When it was reduced to .115 wall thickness, the bore diameter at the muzzle opened up.

Unless its somehow shooting really good, have the threaded portion cut off, re-crowned an either not threaded or threaded 3/4-24 or 5/8-24.

Id go with 3/4-24 to minimize the effects on bore diameter, but 5/8-24 would almost certainly be fine too.

 

[ken226]

Im getting about 1800psi at the muzzle. With a wall thickness of .115" (1/2-28 bore and a .461 minor diameter divided by 2) which results in a hoop stress of 3607psi.

The tensile strength of 416r being about 91,000 psi , you shouldnt even be close to its yeild. So i doubt its a pressure yield issue.

I think the 416r steel had a little compression stress and turning it so thin resulted in the bore material being relaxed a little.

At full diameter it was a consistent diameter. When it was reduced to .115 wall thickness, the bore diameter at the muzzle opened up.

Unless its somehow shooting really good, have the threaded portion cut off, re-crowned an either not threaded or threaded 3/4-24 or 5/8-24.

Id go with 3/4-24 to minimize the effects on bore diameter, but 5/8-24 would almost certainly be fine too.

Also, in your original post you identified the threads as 5/8-28, which is unlikely as that isn't a commonly used standard on firearms. I assumed you meant 1/2-28, but its possible you meant 5/8-24.

Which is it? And, if you push the bushing in past the threads, does it tighten up?

 

[TheBeard413]

Yes I meant 5/8"x24

 

[ken226]

How does it shoot?

If you can determine whether or not its of normal diameter in past the threads, itll tell you if cutting off the threads and recrowning will help.

If its of normal diameter inside, past the threads, then id recommend parting off the threads, recrowning and threading it 3/4-24.

If its still loose inside past the threads, id recommend you contact Bartlein as your barrel is out of spec and should be replaced.

But, if it shoots good, id let it be.

 

[biffj]

I have seen barrels open up when the muzzle was threaded to smaller diameter like 5/8 or 1/2" from something larger like 3/4 to 7/8". Normally though it has only been button rifled type barrels as the buttoning creates stresses in the steel that unload when machined. As far as I know Bartlein cuts their rifling so that is less likely. I suppose its possible that the steel bar the barrel was made from had stress in it. Generally the better barrel makers start out by stress relieving the bar stock the barrels will be made from. The simplest way to tell is to shoot it and see. I've seen a few barrels that have copper fouling all the way up to where the threads start at the muzzle and then a clear area under the threaded part. Others have fouling all the way to the muzzle. In a number of these the barrels shoot poorly but not all of them. So it pays to "give it a shot" as it were.

Frank

 

[ken226]

Yes, properly stress releived barrels shouldn't experience this.

But there are so many unknowns here. A bad stress releaving, post stress relief machining and an infinite number of other possibilities could cause problems.

A >.001 change in land diameter' while extreme' doesnt necessarily mean it wont shoot well provided the groove diameter is good.

I think the op should shoot it, and if it shoots good' leave it alone.

If it shoots like crap, send it to one of the Hides many reputable smiths for repair.

 

[Douglasskid]

Just had a barrel threaded, no measurable belling.

 

[TheBeard413]

I believe somewhere during the process the barrel must have experienced so stress causing it to bell. I have been on the phone with my Smith and he stated he would thread, chamber, and Fit a new barrel for a reduced rate.

 

[FUNCTIONAL]

What about trying a minor back bore to where the thread shoulder is? Theres a reason 22lr match barrels are back bored because when you remove material the bore can slightly relax.

Doesnt have to be a big counterbore...just a smidge over groove size.