To be more clear, I don't want to thread my barrel to 5/8-24 for a muzzle brake because I feel it is removing too much material from my barrel and might affect accuracy.

I have a .308 Rem 700 PSS with a barrel diameter of 0.830" at the muzzle. If my barrel diameter would have been 0.850" I would have been happy with cutting the threads to 3/4-24 to retain more material.

My question is, how about if I thread my barrel to M18-1.5?

I figure 18mm is larger in diameter than 5/8".

Someone smarter than me please advise on which would allow me to retain more material on my barrel.

Thanks!

For only a .5" from the end turn down, I do not see how there could be any loss of rigidity at the end of the barrel. It's not going to change the bore or crown.

Regardless of barrel diameter, when threads are added the specs of the threads will be the same.

So you would have to remove more material for a larger barrel diameter.

Reprofiling the muzzle should have no impact on accuracy or barrel harmonics when done properly considering most of that is established with barrel profile leading up to the muzzle.

I'm only concerned about accuracy because I want to add a brake to a long range rifle. I don't want to make any modifications that could compromise any degree of accuracy, as a shot dispersion of .5" @ 100M would exponentially increase the farther the distance.

None of this matters on my other carbines and rifles, I'm strictly talking about my "precision" rifle.

While researching this I found this video which is influencing my opinion that reducing barrel diameter from .830 down to .625 could  possibly result in accuracy issues.



The way I see it is I'm starting with a .830" diam muzzle. If I cut it down to .625 after threading and then subtract .300 for bore diameter that leaves me with .1625" wall thickness. The unthreaded stock barrel has .265" wall thickness at the muzzle.

If I switch to M18-1.5 metric thread, I'm cutting down to .708 after threading minus .300 bore diam leaves me with .204 wall thickness (compared to .265 stock).

That's a .0415" increase in barrel thickness with M18 vs 5/8". To me that's still more meat on the barrel to help control expansion of a thinner wall.

If there is any validity to the video I posted above, that means the bore opens up for the last 1/2"+ of bullet travel, which to me means the projectile will be deflected from it's original path down the barrel. That should result in a larger spread on paper down range. No?

What about a clamp on...?

As a practical matter at long range wind becomes the biggest variable in dispersion.  Plenty of long range shooters chop barrels (for handiness) and thread for suppressor capable muzzle devices. It isn’t uncommon for those shortened barrels to group the same or tighter.  I imagine benchrest is the discipline that would have the most sensitivity to this concern.  You could try Magnaporting (EDM of ports isn’t without its potential side effects either though).

The video author didn’t say if those barrels got looser after just threading (indicating a stress relieving mechanism) or after firing (indicating some stretch owing to thinner barrel wall).  I have a hard time believing either would create a half thousandth of internal diameter growth.  Video didn’t shoot barrels for accuracy before and after threading.  

Plenty of fine shooting factory barrels threaded 5/8-24 and if they are hammer forged that machining is done post bore-forming. Interesting Video.

His methodology isn't scientific at all and raises more questions with me about what his motive is.

I'm highly skeptical that what he "shows" is actually the whole truth. Even if the results demonstrated in the video is truth, is there any demonstrable evidence that a relaxation of bore size on the order of .00001" has any effect on accuracy?

I do know there are a whole lot of .30 cal precision rifles out there shooting a single ragged hole threaded 5/8-24.

Lastly, from a scientific perspective. If what he demonstrates is true, it is most certainly a result of releasing residual stresses from the manufacturing process when the barrel is machined. That means that different manufacturing processes (CHF, cut rifling, button rifling, post treatments, etc) will all affect the level to which this occurs. Yet he didn't share any of that info. And even profiling a barrel with those residual stresses will change bore size. The question is, how much does it change it and what is considered "acceptable" change? Furthermore, it was evident that all the barrels that showed the "problem" were likely of the same manufacture.

I'm skeptical but won't discredit the video bc I think it's worth more investigation. As always, people can make "data" say whatever they want by delivering it in a specific way. Take things with a grain of salt.

I have a PSS with the same barrel.  I cut and threaded it to 5/8x24 and experienced no loss of accuracy.  Cut with confidence.

The bigger concern is how tight you install the brake.  A drop of blue Loctite and hand tight works best.  Precision barrels don't like brakes tightened by a gorilla.

Why not just do 3/4-24 like you put in the OP?

Not enough shoulder? That is plenty of material for 3/4" threads.

Don’t do it. It will destroy the accuracy. It’s probably already opened up groups just talking about it. C’mon man. ???????

I'm using a Hellfire brake which uses an adapter, but the 3/4" adapter they offer stipulates the muzzle diameter must be a minimum of .850".

Usually I would say I dont see it making a difference, being twenty thou, but they want a specific shoulder size it looks like. It would be worth asking them if a 0.830" barrel would work. I have seen a number of times where a dimension like that is given to a round number for neatness (I have done it) but has some leeway. Im guessing they dont want you to put it on a .76" barrel and try to index off the muzzle.