Barrel threading

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WarrenSmith
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Barrel threading

Post by WarrenSmith » Fri Aug 31, 2018 6:29 pm

Just finished reading these two articles on barrel threading and wondered if anyone has any personal experience or comments to add.

http://gunsmith.co.nz/wp-content/upload ... -57-59.pdf

http://bulletin.accurateshooter.com/201 ... uch-steel/

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Re: Barrel threading

Post by ECCO Machine » Fri Aug 31, 2018 10:37 pm

I honestly don't have the patience to do more than skim those loquacious articles right now, but in a nutshell, there's more than a little bit they're wrong about or not covering well at all.
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Re: Barrel threading

Post by Historian » Fri Aug 31, 2018 11:52 pm

This thread brought up an episode of that long ago
gunsmith RedJacket who on his series believed he could
thread a barrel freehand in the vein of the referenced
YouTube below.

<< https://www.youtube.com/watch?v=2bDAPbibMbU >>

Hmm. After some thought I will not try to practice threading
using this technique the investment Chapuis Savana Double Rifle in .416 Rigby. :lol:

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Re: Barrel threading

Post by WarrenSmith » Sat Sep 01, 2018 12:49 am

Perhaps the question was a little obtuse and required too much reading.

Has anybody measured changes in muzzle bore/rifling diameter after threading?

These two articles are intimating it can happen under certain conditions due to changes in manufacturing induced barrel stresses brought about by removing barrel material during the process of threading.

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Re: Barrel threading

Post by ECCO Machine » Sat Sep 01, 2018 3:46 am

WarrenSmith wrote: These two articles are intimating it can happen under certain conditions due to changes in manufacturing induced barrel stresses brought about by removing barrel material during the process of threading.
It's more a theoretical problem than a real phenomenon. I caught the part where dude claimed a bore @ muzzle was .0015 larger than bore at chamber. Yeah.......did he measure it beforehand? Because my money says it was always that way. In the video, other dude is comparing the bore diameter consistency of button rifled production AR barrels with cut rifled precision barrels. They're never gonna compare favorably, and there's a reason air gauging is done in the accuracy world, even with cut rifled tubes; bore size will vary slightly from one point to the next.

I routinely thread .30 cal sporters at 1/2-28. I caution people that if they're putting rounds down range at a rate that heats the barrel appreciably, the hoop stress could potentially cause belling of the thin walls. But again, these are typically hunting rifles.

5/8-24? Pffft. If it was gonna happen, it would definitely occur on short barreled machine guns. I've had my post sample .308 AR extremely hot after back to back mags, bore @ muzzle is still tight to a .3002" pilot bushing. If 5/8-24 were problematic, it wouldn't be the standard for .30 cal barrels from high end outfits. Look at Proof Research; they have the tenons pre-turned at .625" and left long. These are not cheap OEM-type button rifled tubes.

Let's face it; there just aren't that many barrels which can even take 3/4" threads. Not one sporter I can think of; many of them have to be cut back considerably and/or include a custom collar to interface with the brake or can because the barrel OD is barely enough for 5/8 threads.

Then there was that nonsense in article #1 about threading class 4 or tighter external threads for an interference fit. Any gunsmith who does that is bound to have a pissed off customer when they go buy a brake or DT suppressor and cant screw it down full, or worse yet, can't get it back off due to galling. If someone requests it, I'll do 3A, but a gauge muzzle threads at 2A and undercut all of them .050" so any muzzle device will go on. I've had to chase a couple of factory rifles that were threaded 3A or tighter because they wouldn't take a QD brake.

The most important thing about muzzle threading is axial alignment, which brings me to another eye rolling part of that article when they talk about threading between centers. Nobody calling themselves a gunsmith should be doing that for a weapon that is to be suppressed. Period, end of story. I explained in another thread, but bottom line is, to have properly concentric and axially aligned threads, you use a sized, perfectly true rod at least 4" long that indicates the last 1.5"-2" of bore. If home shop guys wanna thread their own stuff between centers, that's their business, but if a customer is paying ~$100 or more to have a barreled action done, it better be right. Sometimes it takes a little while to dial them in, but that's what we're being paid for! So if you walk into your gunsmith's shop and see some dinky lathe with a 1" through bore and no spider in sight, run, don't walk away from having him thread your barrel for a can.
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Re: Barrel threading

Post by yondering » Sat Sep 01, 2018 4:50 am

WarrenSmith wrote:Perhaps the question was a little obtuse and required too much reading.

Has anybody measured changes in muzzle bore/rifling diameter after threading?

These two articles are intimating it can happen under certain conditions due to changes in manufacturing induced barrel stresses brought about by removing barrel material during the process of threading.
Yes, the muzzle ID can definitely open up after threading, to the point that the difference can be seen in fouling patterns in the rifling.

While I agree that the article from NZ seems to have a lot of information that's purely the somewhat odd opinion of that particular gunsmith, the Accurateshooter article is spot on, and Robert Whitley knows what he's talking about here. I'm guessing from the comments above that ECCO's personal experience doesn't give him the context to fully understand that, but it's long been known that cutting the OD of a rifle barrel smaller (whether in threading or just profiling) can tend to open up the ID. The differences are small, but they are there, and there's good reason that guys winning accuracy competitions these days have taken steps to address the issue. I doubt any of ECCO's 1/2-28 threaded sporter barrels are shooting groups in the ones, for example; totally different context and in those cases the differences are small enough that they probably don't matter for that application, but it doesn't mean they don't exist.

So yeah, for best accuracy it's a good idea to go with the largest thread you can use. For very thin barrels, I actually prefer to make the muzzle device shoulder up against the muzzle if possible, allowing a larger thread diameter than if the muzzle were cut down to provide a shoulder. I've turned away threading jobs where people wanted 1/2-28 on thin sporter 30 cal barrels for suppressor mounting, I won't do it, but that's just me. In my experience there's usually a better option.

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Re: Barrel threading

Post by ECCO Machine » Sat Sep 01, 2018 1:10 pm

yondering wrote: I'm guessing from the comments above that ECCO's personal experience doesn't give him the context to fully understand that, but it's long been known that cutting the OD of a rifle barrel smaller (whether in threading or just profiling) can tend to open up the ID.
My point wasn't that you can't minimally affect bore diameter or even straightness by reducing OD, but that bore diameter consistency in general and the behavior of cut rifled barrels when machined is quite different from hammer forged or button rifled. While I haven't personally conducted any testing, I'd bet the difference is less dramatic with the two different thread diameters on identical cut rifled barrels.

With button rifled or forged barrels, it's pretty easy to feel the variations in bore diameter just over the couple inches I insert my indicating rods. When I rebarrel something with a Schilen, Lilja, Bartlein, Kreiger, etc., that tight-loose-tight loose insertion tends to be absent. It is also important to remember that we're talking about a couple of tenths at most, it's not like OEM barrel bores are a series of hourglasses where the bullet is out of contact part time.
yondering wrote:. I doubt any of ECCO's 1/2-28 threaded sporter barrels are shooting groups in the ones, for example; totally different context and in those cases the differences are small enough that they probably don't matter for that application, but it doesn't mean they don't exist.
No, these are hunting rifles that can probably mechanically produce sub-MOA easily but that the owners really only care about being minute of vital zone at the ranges they shoot. Nobody trying to win competitions with whippy pencil barrels!

That said, many folks report an improvement in accuracy when we shorten & thread. These guys want to keep OAL down with a can attached, pretty frequently remove 2" to 6" before threading.

I do quite a few "precision" rifles, but no, I don't have a competition benchrest clientele. I see rather few rifles through here with fat enough barrels to go bigger than 5/8.
yondering wrote: For very thin barrels, I actually prefer to make the muzzle device shoulder up against the muzzle if possible, allowing a larger thread diameter than if the muzzle were cut down to provide a shoulder.
My approach for max thread size is a custom collar to provide shoulder

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Re: Barrel threading

Post by John A. » Sat Sep 01, 2018 1:27 pm

That is a beautiful thread job. I don't recall ever seeing anyone go so far back over the barrel before the threads start though.

I tend to do a relief cut next to the shoulder though. You'd be surprised how many muzzle devices won't thread that last 1/4 of a turn without a relief cut slightly below the threads. I like the device to butt squarely against the shoulder. And with threads all the way to the shoulder, does occasionally cause a problem with some things.
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Re: Barrel threading

Post by ECCO Machine » Sat Sep 01, 2018 1:59 pm

John A. wrote:That is a beautiful thread job. I don't recall ever seeing anyone go so far back over the barrel before the threads start though.
Undercutting the front helps with axial alignment (less risk of cross threading with a short minor diameter pilot), and also protects the threads better in the absence of a protector. Looks better, too, IMO.
John A. wrote:
I tend to do a relief cut next to the shoulder though. You'd be surprised how many muzzle devices won't thread that last 1/4 of a turn without a relief cut slightly below the threads. I like the device to butt squarely against the shoulder. And with threads all the way to the shoulder, does occasionally cause a problem with some things.
It's the termination of a thread just before the shoulder that causes that problem.

I always undercut the rear with normal thread jobs. With the collars, though, the threads are continuous. The thread tenon on that rifle is actually .500" longer than what's visible; the collar is threaded onto it (with Rocksett), cranked as tight as possible, then profiled.

Different barrel but same process:

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Re: Barrel threading

Post by 300sniper » Mon Sep 03, 2018 5:59 pm

For muzzle threads where the customer needs to be able to attach anything, I am threading to a spec, which with my machinery, I am able to hold the PD in a tight spec where 3A and 3B overlap I am not sure why suppressor manufacturers in NZ don't thread to a spec. Seems a bit odd to me, if not flat out misinformation.

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