Threading a barrel

[jmol50]

I'm looking to cut my Bergara B-14 in 308 down to 16 inches and thread it. I dropped it off with local gunsmith yesterday and he said no problem, he'll just have to thread it 1/2-28 and put on an adapter to get to 5/8-24. I didn't think anything of it yesterday but now that I think about it I'd prefer a direct thread and not having extra pieces if possible. It's a thick barrel (contour #4?) so not sure why he's cutting to 1/2.

Should I call my gunsmith to ask why 1/2?
Should I call another gunsmith to see if they'd do 5/8 thread?
Are thread adapters not an issue?

 

[NikA]

I'd call and indicate you prefer the larger thread size. Probably not an issue if your smith is well outfitted. If you're intending to run a suppressor on it, make sure he knows that as well since it has implications for the thread class and perpendicularity to the bore.

Thread adapters can work but you would be correct to avoid them in applications where tolerance stacking might matter. Sometimes that's not possible in which case they should be attached as permanently as is practical (hi-temp thread locker, etc.).

 

[jmol50]

Thanks @NikA - I'll give him a call to check in about it. I can also share the suppressor info with him.

 

[Mobuck]

YES, you should question him on this. Cutting a .308 bore down to thread @ 1/2x28 doesn't leave much 'meat in the muzzle'. It IS possible to 'bell' the muzzle if the metal is too thin.
It's not just the threaded area that is an issue, it's also the 'shoulder' created when the barrel is threaded that supports a suppressor (if that's the reason for threading). If this is just intended for a muzzle brake of flash hider, then less of an issue.
I recently had a .308 barrel threaded and chose to use a 9/16x24 thread as a compromise. This allows a bit more meat in the muzzle while providing a suitable shoulder at the back of the threads. It does cut down the options for a muzzle device but I'll accept that rather than risk a muzzle split.

 

[jmol50]

Thanks @Mobuck. You and @NikA have given me the confidence I needed to "question a professional".

 

[Hvymax]

The "gunsmith" probably only has the one tap and an annular cutter instead of an actual lathe. He'll probably wrap duct tape around the 22cal pilot for the cutter.

 

[Mobuck]

^^If so, I wouldn't trust that at all.
Unfortunately, I lost the best shop for such work in the area(if you can call 125 miles away 'in the area'). I guess other opportunities became more lucrative and his talents went down a different road.

 

[Hvymax]

There's surprisingly little money in gunsmithing these days. The time to set up a $100 sight dovetail cut and the equipment makes it nearly minimum wage.

 

[Dave P.]

I'm not a smith but if I had to get to 5/8 x24 I might thread the barrel to whatever I wanted, then thread the ID of a
blank round with a bit of interference fit and tighten it to the barrel, maybe with a slight over size at one end
to accept a bit of solder. Once it was installed then thread and shoulder what is now the adapter to the 5/8 x 24,
add a crown to it and call it done. For aesthetics the end of the "adapter" at the barrel could be done a few ways
depending on how you wanted it to look.

 

[Hvymax]

I'm not a smith but if I had to get to 5/8 x24 I might thread the barrel to whatever I wanted, then thread the ID of a
blank round with a bit of interference fit and tighten it to the barrel, maybe with a slight over size at one end
to accept a bit of solder. Once it was installed then thread and shoulder what is now the adapter to the 5/8 x 24,
add a crown to it and call it done. For aesthetics the end of the "adapter" at the barrel could be done a few ways
depending on how you wanted it to look.

That would require an actual Smith with good tooling.

 

[Mobuck]

^^Probably more difficult than actually threading. Biggest problem with turning the outer diameter is the fact that the bore may not actually be centered in the muzzle. Cutting threads based on the bore will correct this.

 

[Dave P.]

^^Probably more difficult than actually threading. Biggest problem with turning the outer diameter is the fact that the bore may not actually be centered in the muzzle. Cutting threads based on the bore will correct this.

That was basically my point, if he needed the larger thread size ...ie. don't thread the OD of the adapter till it was permanently installed
to the barrel, then indicate the bore and alignment and cut the 5/8 x 24 thread and shoulder. Doing it that way you essentially get the same
concentricity and alignment as you would get by threading the barrel itself if it had sufficient diameter to start with.
Yup, more work, precision always is for custom crap. I don't know enough about silencers to know if it actually matters if they're
off a few thou. so maybe no gain for the extra work but it still seems the right way to do it.

 

[Dave P.]

That would require an actual Smith with good tooling.

Not much other than a decent lathe ( even a crappy one if you knew it's issues ) and the decision
to do quality work.

 

[Mobuck]

There's large selection of ready made 5/8x24 to 1/2x28 adapters. Fewer but still available are 9/16x24 to 1/2x28. Less likely is something like 9/16x24 to 5/8x24 due to thin wall of adapter.

 

[krw]

It wood have really helped this thread if you wood hav taken a caliper and measured diam of bbl at 16"

 

[DutchV]

My usual guy wants a minimum diameter of .700" for cutting 5/8-24 threads. I'd much rather have the actual thread size than use an adapter.

 

[Boxhead]

There was a time, not long ago, where folks were always chasing velocity and wearing hearing protection. Not so much anymore it seems.

 

[Mobuck]

"minimum diameter of .700" for cutting 5/8-24 threads"
Even that only leaves .0375 of 'shoulder' to support the muzzle device. This isn't much and is why I chose to go with the 9/16x24 thread which gives a slightly better .06875 shoulder.

 

[jmol50]

It's cut and threaded with adaptor. Looks OK enough to me. Thanks for the advice.

 

[Hvymax]

How much did they charge?