Threading barrel for muzzle brake

[sbd3927]

Just an opinion, but it doesn't seem to be much of a concern.
5/8 = 0.6250" =15.875
ANSI Standard, not listing 24 pitch thread though...
http://www.engineersedge.com/screw_threads_chart.htm
0.6250 nominal for 18 pitch, 0.6163 minimum = .6163x25.4 = 15.625
If you look higher in the chart for a matching 24 pitch thread, the tolerance between max & min is .0070" on a 3/8" or 0.17mm
that puts your .075mm better than halfway in the 0.17mm tolerance.

Another way to look at it, 15.875 - 15.8 = .075mm, =75 micron.. = fine human hair, and that's on a round thread, so it's half a hair depth on each side.
I doubt the gunsmith would machine it to that tolerance, if you cut to the correct specified depth, it would be equivalent to miniscule wear on the outside (non working part) of the thread. Finally it's more about the thread pulling it into the shoulder and sitting true to the bore.

Hope this helps.

[zobster]

That's exactly my concern, there's hardly any material for threads to be cut properly and it's putting a lot of faith in the smith's skills. Think I might just go with 1/2*28 threads. Its common enough and I could always bore out a 223 muzzle brake to fit a 308.

[sbd3927]

I'd go with the 5/8 for the same reason, the difference is insignificant, the barrel may well be thicker or thinner by that amount already. The effects on the crown outlined by Homer above seem far more important.

https://en.wikipedia.org/wiki/Unified_Thread_Standard
Look at the thread picture, the external thread already has a flat on top, and the barrel is within the tolerance for thread, the flat would be very slightly wider.
The chart has the 24 pitch spec, 5/8 UNEF. Subtract the tapping drill (14.68mm) size from the external, the overall thread depth is 1.19mm

You could get a bolt of that size (specialised bolt shop), hold a hair against it and see how little difference it would make.

Alternatively, go for a clamp on style brake?

[Chickenhawk]

Thinning the barrel wall through threading it is an interesting concern.

Does anyone know if someone like SAAMI provide a standard a standard on this or is it completely up the to the knowledge/skill of the gunsmith?

[Oldbloke]

Thinning the barrel wall through threading it is an interesting concern.

Does anyone know if someone like SAAMI provide a standard a standard on this or is it completely up the to the knowledge/skill of the gunsmith?

Pressure will have been well and truly reduced by the time the projectile gets to the end of the barrel. Don't worry.

[Xerox]

Hmm, it's an worthwhile question though.

If a bullet lodged at the muzzle or entering the brake for some reason I suspect there would be more than enough pressure to still octopus the thing in anything but the smaller calibres.

.223 may be relatively safe, surely not a .30 though.

[zobster]

Hi Guys,

I've bit the bullet and have sent the gun in for threading.

Was told the following "Your barrel is a sporter profile, I can cut a 5/8*24 thread on it but there isn't any shoulder for the brake to screw onto".

Please help, what is my next move? I really want to use the brake.

[bladeracer]

Hi Guys,

I've bit the bullet and have sent the gun in for threading.

Was told the following "Your barrel is a sporter profile, I can cut a 5/8*24 thread on it but there isn't any shoulder for the brake to screw onto".

Please help, what is my next move? I really want to use the brake.

Use a locknut.

[zobster]

Problem solved! Smith told me to get 223 brake in 1/2x28, he'll bore it out to 308, no charge!!