Muzzle threading vs barrel diameter

[KU_Geo]

i'm looking to have a couple guns threaded and have two different smiths give different guidance on what is acceptable threading size. All three guns measure .625" at the muzzle. Calibers are 6.5mm, 7mm, and .308. Should I go 1/2" or 9/16" on these? Plan to use a self timing brake in all of them and do not plan to contour to same diameter as the barrel

 

[J E Custom]

Here is my recommended barrel sizes for barrel threads based on Minimum safe barrel wall thickness.

http://www.jecustom.com/index.php?pg=Home&cmd=Cell&cell=FAQ

The last topic in the FAQ section will give you a recommendation.

J E CUSTOM

 

[FEENIX]

i'm looking to have a couple guns threaded and have two different smiths give different guidance on what is acceptable threading size. All three guns measure .625" at the muzzle. Calibers are 6.5mm, 7mm, and .308. Should I go 1/2" or 9/16" on these? Plan to use a self timing brake in all of them and do not plan to contour to same diameter as the barrel

I have two (.300 WMs) similar to yours and both have 1/2-28 thread, one was Holland's QD brake and the other is similar to Holland's except with 4-ports. Call the MB brake maker you are going to use and ask them directly, i.e., Nathan of https://muzzlebrakesandmore.com/beast-2-piece-brakes. I have one of his 4-port 2 piece beast brake except in 5/8-24 for my .264 WM and it is simply awesome. Good luck!

 

[IdahoCTD]

With self timed brakes or brakes that need to be tightened hard against a crush washer you need as much barrel wall thickness as you can get to try and avoid distorting the bore when the brake is tightened. In your case I would do a 9/16x24. The 6.5mm wouldn't be as much of a concern but I would try to be easy tightening the brake on a 7mm and .30. I only use 1/2x28 threads when I have no other choice on larger calibers, not as a first option.

 

[KU_Geo]

Good info guys. Looks like 9/16" might be the way to go, and fine for the smaller calibers. I plan to use MBM brakes so will give them a call as well.

 

[Edd]

https://www.longrangehunting.com/threads/brake-for-small-contours.214855/#post-1565164

 

[J E Custom]

Just an adder to the previous post.

I prefer .150 thousandths barrel wall thickness at the minor diameter of the thread for muzzle brakes because of possible distortion of bore and safe pressure containment. less will work, but only with fitted threads and proper installment.

Barrel makers will say no less than .137 (That an exact engineering dimension) that will cover all pressures generated by different cartridges.

1/2 28 TPI has been used on 30 cal for a long time but even the military has gone to the 5/8 x 24 TPI for safety. Of the muzzle brake failures I have seen, 1/2" x 28 have made up the majority for thread failures. Other failures have been improper installation.

Even threads that have a minor diameter larger than .150 more than the bore wall thickness are recommended when possible to lessen the chance of failure and disturbing the bore diameter.

There are a lot of things that are done and have not caused problems. but how close have they came to catastrophic failure is something we never find out if we are lucky.

J E CUSTOM

 

[arch408]

I use 9/16 X 32 threads on larger caliber barrels that aren't big enough to use a 5/8X28.

 

[IdahoCTD]

Most people dont realize this but there is only about .005" difference going from 24tpi to 28tpi and .005" difference going from 28tpi to 32tpi. That is the total minor diameter change. Split that in half and that is how much more barrel wall thickness you get. So from 24tpi to 32 tpi it adds roughly .005" per side. IMO that is hardly anything when your talking .150" being a safe minimum of barrel wall thickness. Somehow I doubt 3% more barrel wall thickness is going to save the day on a distorted barrel from over tightening or from ripping that threads off the barrel. Even in the case of 1/2x28 tpi on a .308 barrel if you take the 1/2x28 absolute minimum diameter (.4564" on 2A threads or .4572 for 3A) and subtract .308 that gives you .148/2 or .074" of wall thickness. Going from 28tpi to 32tpi adds roughly .0025 per side or in the neighborhood of 3%. Not a magic pixie dust fix for having to run small thread sizes on a skinny barrel and not worth limiting myself to running only 32tpi brakes IMO.

 

[jimirwin]

Here is my recommended barrel sizes for barrel threads based on Minimum safe barrel wall thickness.

http://www.jecustom.com/index.php?pg=Home&cmd=Cell&cell=FAQ

The last topic in the FAQ section will give you a recommendation.

J E CUSTOM

Your recommendation says the standard for .308 muzzle threads (5/8-24) is unsafe. Somehow this doesn't hold water for me.

 

[J E Custom]

Your recommendation says the standard for .308 muzzle threads (5/8-24) is unsafe. Somehow this doesn't hold water for me.

.648 (Recommended minimum) leaves .135 Wall thickness. This ls just below the minimum engineered wall thickness of .137

Not only is the 5/8 x 24 safe, but recommended. anything less will cut into the barrel wall thickness and depending on the cartridge intensity could be dangerous in my opinion. As stated I recommend .150 wall
thickness but you can use the engineered minimum and would still be ok depending on thread fit and the amount of support offered by the muzzle brake design.

You can go to a 28 or 32 TPI and increase the wall thickness by a small amount, but most of the accessories are 5/8 x 24 TPI.



J E CUSTOM

 

[arch408]

Another option if the muzzle is big enough to cut 5/8X24 threads is to have the inside of the brake butt up against the muzzle, but it makes clocking the muzzle brake trickier. To do that, I make a blank brake body that butts up against the muzzle. Then I can secure the barrel in a vise on a mill and then bore the ports where I want them.

 

[J E Custom]

Another option if the muzzle is big enough to cut 5/8X24 threads is to have the inside of the brake butt up against the muzzle, but it makes clocking the muzzle brake trickier. To do that, I make a blank brake body that butts up against the muzzle. Then I can secure the barrel in a vise on a mill and then bore the ports where I want them.

We time the brake this way also, when there is not enough barrel shoulder to seat the brake on. I do the Timing of the brake the same way except on the barrel end instead of the shoulder.

I also bore the thread relief the same size as the barrel enough to clear the threads so it looks seamless.

Different way to accomplish the same thing

J E CUSTOM

 

[arch408]

I left out that part, but it does make neat looking brake.