Re“Muzzle Threading — Don’t Remove Too Much Steel”

[pgcpty]

To the Gunsmiths out there.

I'd like advice on this one. A year ago I had a Harrell’s Muzzle Brake fitted to my wife's 308 Remington 7600. Subsequently group sizes opened up and whereas before fitting the brake the rifle would hold 1/2 MOA, after fitting the brake this deteriorated to in excess of 1MOA.

I suspect this was caused by the muzzle diameter expanding due to removing metal to thread for the brake. The brake tenon 0.5" DIA X 32TPI X 0.5"long leaving 0.078" approx radial wall thickness over the bore diameter. The bullet clearance DIA through the brake is 0.328" (21/64")

So the question is, will back boring the muzzle diameter by the length of the brake engagement cure the problem?

If so how much clearance over the nominal groove diameter of 0.308 has been found to be adequate?

Thanks for your help.

 

[Scott Harris]

It might work. Only concern is leaving too little metal on the external barrel thread. Minor diameter of a 1/2 x32 external thread is around .462. If you bore out the barrel underneath it to .328, that leaves about .067"thick threaded section. I'm not sure what is considered the minimum acceptable thickness. Is this a stainless steel barrel? Krieger won't make a .30 cal stainless barrel with a muzzle less than .630" or so. So, obviously, they would not recommend a 1/2x32 thread on one. I would just cut the threaded section off and re-crown if the choice were mine.

 

[Dusty Stevens]

It may be a poor crown. Id lop it off with a good crown and shoot a lighter bullet

 

[carlsbad]

The problem with boring out the barrel beneath the threads is that it will be very hard to create a good crown. The bottom surface of the bored section will be your new crown. The best way (that I can think of) to get a good crown would be to bore it, make a lapping mandrel a few thousandths smaller and lap the new inner face. Time consuming and expensive. If I were going to do this I would try to bore to the same diameter as the muzzle brake if that is adequate.

I should mention that there are other things that could have caused the barrel to perform poorly besides the current theory of "don't remove too much steel". although I like the concept of leaving the wall as think as possible at the muzzle, I'm not sure it really holds water.

Other things known to affect accuracy: 1. Bad crown: did the gunsmith recrown the muzzle under the brake? If not, did he ding the muzzle and fail to repair it? 2. Poorly centered muzzle brake. To properly install a muzzle brake the barrel should be set up in the lathe with some sort of range rod or "grizzley rod" in 3 dimensions just like a chambering job so that the muzzle brake is aligned with the bore, not with the OD. Some lazy gunsmiths just put it in a 4 jaw chuck and indicate the OD of the barrel. 3. Harmonics. I believe Erik makes a proprietary tuner brake that might fix your problem. 4. I'm sure there are more but it is late.

Looks like Scott just posted to cut it off and re-thread. I agree with him. I might go one further and just rebarrel. If you're after sub .5 moa accuracy, spend the money on a new barrel and use this one for another project or sell it.

--Jerry

 

[lawrence97]

Or you can screw on a Harrell's tuner brake and tune out any vertical after proper load development.

 

[gunsandgunsmithing]

I've seen a .338 win or rum(I don't recall which), that someones else put a brake on that was threaded 1/2-28. The barrel very obviously swelled under the threads to the point that you could easily feel it with a patch and even clearly see it. There was zero doubt here.


Of course a big magnum case, slow powders and a .338 bore are quite different than your situation, but it was very plain to see what had happened. I've still got the barrel stub somewhere, that I cut off when fixing that mess.


Muzzle pressures can vary a great deal with barrel length, powder volume and burn rate, as well as other reasons. In this case, the pressures were simply way more than the thin metal under the brake could or should stand. This is an extreme example of what really happens every time we fire a gun...The barrel grows. That's how strain gauge pressure systems work, by converting that stretch to current that can be given numeric value.


Just thought I'd throw that out there, as it's often wrongly assumed that the barrel miraculously doesn't change under these pressures. It shines a new light, to some, on why brass can and does get tight after repeated or over pressured firing. Both brass and steel are somewhat elastic and will essentially return to their nominal size until taken beyond it's yield point, OR repeatably stressed to near it.

 

[pgcpty]

Having read all your comments, I've decided to try boring out the Brake tenon to 0.328" matching the Brake bore diameter leaving 0.067" radial wall thickness for the 0.5"length of tenon, then lap the crown as suggested. Don’t envisage problems with the reduced barrel wall thickness because the gas pressure will drop dramatically immediately the bullet exists the new recessed crown and 0.067” wall thickness is comparable with many auto pistol barrels fitted with moderators.

Thanks for all your inputs, makes all the difference!

 

[CaptainMal]

STOP!

You have or had a Remington 7600 in 308 that holds 1/2 MOA? I read that right?

A Remington 7600 is like my 760 pump carbine. I spent decades to get maybe a reliable 1.5 MOA out of the thing. Not sure that level is reliable for over say 2-3 shots. The barrel hangs on a flimsy action, has no bedding, a multi-lug bolt crudely stuck into that action and maybe pieces of pump action touching that barrel.

If you now can get 1 MOA you still have a jewel compared to most every other one I have seen, and that's a lot. Most always 2 - 3 MOA is typical with a few exceptions over the decades. They are Minute of Deer guns.

Good luck with "fixing" it. I say it's more than fixed at 1 MOA.

 

[fredo]

I have to agree.
After having shot several 760s, 742s & 7600s while zeroing scopes for friends, not one would hold even close to a marginally decent group @ 100yds. "Minute of grapefruit" was about average.

If your wife's braked 7600 holds ~1moa consistently, LEAVE IT ALONE and enjoy feeding your semi-auto unicorn. Don't sweat the small stuff!!!