Concerned this might be unsafe

[Bobson]

I’ve got a Tikka 308 with a muzzle diameter of 0.650. I’ve done some research and evidently that diameter is insufficient to safely have the muzzle threaded 5/8-24. I learned that I can have my barrel threaded 1/2-28 and use a 1/2-28 to 5/8-24 adapter, and this seems like a good solution. I’ve also read that many people do this primarily for the purpose of threading a 30 cal can onto a sporter weight barrel, which is similar to what I have in mind. I’m guessing that’s safe, on its own, given the number of times I read about it being done for that specific purpose.

The difference is that I don’t want to use a direct-thread can, but rather a Surefire SOCOM 762RC2 which I would move between the Tikka (after adding a Surefire muzzle device to the Tikka via the thread adapter) and a different rifle that already hosts its own Surefire muzzle device as a SOCOM adapter.

Is there any reason this might be unsafe, provided I’m using a high quality component thread adapter and the barrel was properly threaded by a competent gunsmith? I guess it boils down to me not knowing whether it matters if the suppressor is a direct thread type vs using the Surefire fast-attach method designed into the SOCOM cans and muzzle devices.

I don’t have firsthand experience with any suppressor yet. Got my feet wet by ordering my first Surefire RC2, and now while I’m waiting on the ATF to play their games, I’m wanting to get the ball rolling on the next one.

I really appreciate any advice. Just want to be safe and do things properly. If the answer is that this just isn’t safe and there’s no good way to do it without going to a bull barrel, maybe that’s what I’ll do. But I’d really prefer not to if it isn’t necessary.

 

[DrDeFab]

I'm no expert, and I understand that .650 wouldn't leave an adequate shoulder, but the SureFire adapter doc ( https://assets.surefire.com/uploads/2019/07/71-01-997_Suppressor-Adapter-Installation_Rev-D-1.pdf ) says it is OK to time the adapter with shims at the muzzle. Given that, .625 or larger should be fine at 5/8-24.

 

[entropy]

@MachIVshooter , what say you?

 

[MachIVshooter]

@MachIVshooter , what say you?

Couple options:

1, straight up 5/8-24 threads with a collar:



2, Thread 9/16-24 and run a timed thread 9/16-24 to 5/8-24 adapter if the desired muzzle device was not available in 9/16.

https://www.brownells.com/rifle-par...read-adapter-9-16-24-to-5-8-24-prod86836.aspx

If the barrel OD were much less, though, I'd go 1/2-28.

 

[Bobson]

I’m sorry, I don’t understand option 1 (eek). What do you mean by “with a collar?” Are those in the pic not threaded adapters?

I sent a message to the smith asking about the 5/8-24 threads, but haven’t received a response yet. It seems like 9/16-24 would be preferred over 1/2-28? I really don’t even understand how those numbers are represented in the threads themselves, to be honest. I need to do some more research…

I do appreciate the responses, for sure! THR members have never failed to be able to answer questions during my decade here.

 

[jmorris]

I’ve done some research and evidently that diameter is insufficient to safely have the muzzle threaded 5/8-24. I learned that I can have my barrel threaded 1/2-28 and use a 1/2-28 to 5/8-24 adapter, and this seems like a good solution.

There are two diameters to be concerned with on a barrel, ID and OD. If the bore or ID is too close to the “minor” diameter of the thread, so the wall thickness is too thin, it will break off at that point. This is one reason you see very fine threads a 5/8-32 doesn’t cut into the barrel the same depth as 5/8-11. Then there is the fact that the can needs to be square on the barrel, so the bullet, once leaving the barrel, doesn’t hit parts of the can as it passes through. If it’s crooked, bullet hits baffles. The shoulder is what does this.

I sent a message to the smith asking about the 5/8-24 threads, but haven’t received a response yet. It seems like 9/16-24 would be preferred over 1/2-28? I really don’t even understand how those numbers are represented in the threads themselves, to be honest.

The fractions in 5/8-24, 9/16-24, 1/2-28 are all in reference to the major diameter, or what size hole the threads could pass through. The number following the fraction is the pitch of threads, it is represented as the number of threads over one inch in length, the higher this number is, the finer the threads are.



https://www.trfastenings.com/products/knowledgebase/thread-geometry/unified-coarse-unc

 

[Bobson]

Thanks a bunch, that’s super helpful

 

[BigBlue 94]

9/16-24 is used on AR15s with a 6.5mm bore. There are not many muzzle devices available. Most are just basic flash hiders. Mine is a basic pepper pot style.

1/2-28 is way too small for 30 cal imho. An adapter would help, but every device you thread on the barrel stacks tolerances. You could get slightly off center by the time you slap a can on over a muzzle device over an adapter over threads all cut on different lathes.

 

[MachIVshooter]

I’m sorry, I don’t understand option 1 (eek). What do you mean by “with a collar?” Are those in the pic not threaded adapters?

No, they're not adapters. Those barrels were just large enough to support 5/8 threads, but would have had insufficient shoulders. So we thread them further back than the finished tenon needs to be, screw a 3/8" thick a collar on with Rocksett and really tight, then profile that collar and face the front of it to create a shoulder.

It seems like 9/16-24 would be preferred over 1/2-28? I really don’t even understand how those numbers are represented in the threads themselves, to be honest. I need to do some more research…

If the barrel can support it, yes. As a general rule, you want the largest (common) thread size possible on your barrel to minimize bore growth and belling.

As JMorris explained, the first number, which may be denoted fractional or decimal, indicates the major (crest) diameter of the male threads, the second number is the pitch.

For inch threads, as explained, the pitch is denoted as threads per inch. Strictly speaking, "pitch" is the distance between threads, so a 24 TPI thread is technically a .041667 pitch, but that only matters to us machinists programming CNCs that don't speak in TPI.

Metric threads are denoted the same with regard to diameter, but the pitch is actual pitch; M14x1.0 threads have a 14mm major diameter and 1.0mm between threads.

The other aspects of thread diameter are minor (the root of a male part/crest of a female) and pitch, which is basically the middle of the thread. Pitch diameter is the most critical one, it's how we actually measure them for QA. Major diameter of a male part will also typically be slightly under the nominal thread size, so remember that when measuring. Thread callouts are not for dead sharp crests and roots, so a 5/8-24 thread, nominally a .625" major, may have a measured diameter more in the .619-.622" range. Likewise, female thread minors will be larger than major diameter minus the double depth of a sharp V.

There are also many different types of threads. Unified inch/ISO metric are your standard 60° threads most commonly encountered, but there are also J threads (modified UN/ISO), 55° Whitworth, ACME, Square, Buttress, pipe threads and others. Some of these may be encountered on the breech end of a barrel, but muzzle threads will pretty well always be UN/ISO.

You will, however, encounter left hand (LH) threads on some gun barrels, namely European stuff. AKs are typically found with M14x1.0LH threads, Euro 9mm will frequently have M13.5x1.0 LH and 16x1.0 LH on some .45 cal. And for others they'll do M14x1 RH, M15x1 RH, 16x1, 18x1 and so on. Yeah, it's lots of fun trying to make and stock all these adapters, times as many different male adapter thread configurations.

About the only LH inch threads found on muzzle are the FAL with 9/16-24 LH, though there are a few suppressor muzzle devices with LH threads like OSS and the Dead Air Xeno. Personally, I hate LH threads, only thing they do that a RH thread won't on a gun barrel is piss off smiths & machinists.

Jmorris also covered one of the reasons muzzle threads are generally fine pitch with respect to minor diameters, but there's another reason: Torque vs. pressure. You apply the same amount of torque with a finer thread, you get a lot more pressure on the mating surface, just like any kind of mechanical advantage, leverage. That helps keep cans from getting loose.

Anyway, there's a lot to threads, and I can go into more detail about it if anyone is interested, but I reckon this will suffice for these purposes.

ETA:

I did skip over another important aspect of muzzle threads; the undercut. An undercut on a male thread (visible on the two barrels on the right side of my photo) is a smooth termination at or below minor diameter, and it needs to be there so that muzzle devices which lack a counterbore (I'll cover that in a sec) won't bottom out on threads before contacting the shoulder and wind up canted. The exception is when running a collar like you see in my images, because the threads continue under the collar, so there is no unthreaded portion at the shoulder a muzzle device may encounter.

A counterbore is to female threads what an undercut is to male, a termination at or above thread major. Done for the same reasons.

Chamfered counterbores or undercuts (and chamfers in general) are also used for cleaner thread terminations, and to help with alignment, guiding the threads. It is very difficult to get two parts which have a 90° termination and the jagged partial threads that result to mate, especially with finer threads.

 

[Bobson]

That’s more info than I could have hoped for lol. Thanks a bunch.

 

[Hugger-4641]

I just installed a brake on my Howa 30-06. Had the same issue, my barrel O.D. at the muzzle was only 0.619. I had it threaded to 1/2 x 28 and used a 5/8 adapter and shim kit, works just fine.

 

[BBBBill]

...straight up 5/8-24 threads with a collar...

^^^ That is the solution that I would opt for if I were in your shoes. Elegant, simple, clean...

 

[rbernie]

Those barrels were just large enough to support 5/8 threads, but would have had insufficient shoulders. So we thread them further back than the finished tenon needs to be, screw a 3/8" thick a collar on with Rocksett and really tight, then profile that collar and face the front of it to create a shoulder.

And this works very well - the collars are quite solid and provide an excellent shoulder for the can. It’s a great solution.

(Nick - are those my barrels?)

 

[BigBlue 94]

That’s more info than I could have hoped for lol. Thanks a bunch.

About the only thing he didnt cover is thread class. This has to do with the tolerances the thread was cut to. There are 3 classes, with B being the normal fastener class. Class A is a loose fit on the threads, Class C is a tight fit. Neither are common in the consumer world, but more so in manufacturing.

I learned to thread on a lathe back in 2013. Single point, with a self ground tool, on a 1950s engine lathe with tons of backlash. Those threads were garbage. But they were threads. Its curious how a few thou will totally change the thread fit or even concentricity. The section on threading in the Machinist's Handbook is about a half inch thick, on paper so thin you can see through it.