diameter vs thread size

[cohunt]

ok-- lots of mixed info on the web about this so thought I'd get a few professionals to give me some valid info before looking for a local smith to do the job

minimum muzzle diameter for threading a barrel is the question at hand
I would love to thread each one 5/8x24 but thats just not gonna happen, purpose is to mount either a brake or suppressor. I know that it can be done 1 of 2 ways, indexed off a shoulder or indexed off the muzzle-- I have head that indexing off the shoulder is best for accuracy.

X-bolt in 7 mm--factory threaded for comparison --MuzzleDiam=.728" threaded in 5/8x24 for 7mm

rem 700 in .270cal not threaded yet-- muzzle diameter is .661" maybe find someone to do a 5/8x24 but most likely has to be a 9/16 thread--right?

win 70 in 7mm--not threaded, MD=.605"-- too small for 5/8 for sure, maybe possible 9/16" but probably should be 1/2"? right? but what about "wall thickness" at the muzzle, would this have issues with muzzle swelling?

last one--I'm thinking it's just too small to thread at all--correct me if I'm wrong
it's a rem700 with a MD=.553" in 30 cal--- not only might there be a problem with the thin shoulder if threaded 1/2x28 but probably not enough wall thickness for a 30 cal when threaded 1/2"--right ? would 7/16x28 work on this one like kimber uses?


BUT here is my quandary--- Kimber threads it's subpaline barrels with a 7/16x28 thread pitch-- they sell the rifle in 280ai, 308, 30-06, 300wsm, & 300 wm

how are they doing this with out having issues on those "big" calibers with the thin barrel wall at the muzzle? what is the actual muzzle diameter of those kimbers?

I know it's also gonna depend on what a smith is willing to do, but knowledge first helps me make the right decision as sometimes $$ will buy anything even if it's not the best idea!

 

[J E Custom]

The best way I can tell you for safety is to = Take the barrel diameter and subtract the bore diameter and the minor diameter of the thread pitch. If the barrel wall is over .150 thickness each side, your OK. if it is under, you are getting into the safety factor.

There are thread tenons on some applications that are much less than .100 thickness between the thread root and the barrel grove but these installations are relying on the thread fit and brake for barrel strength.

Some get away with it and some don't. so I recommend that after you cut the thread, you end up with at a very minimum of .125 barrel wall thickness from the thread root to the bore grove.

Hear is a link that will help explain what you ask. Look at the last article under FAQs.
http://www.jecustom.com/index.php?pg=Home&cmd=Cell&cell=FAQ

J E CUSTOM

 

[cohunt]

So what are your thoughts on those 7/16" threads on the Kimber 300wm? Wouldn't that only leave about .040" at the muzzle?

You are basicly saying all 308's should be 5/8 thread or larger

With your calcs a 5/8× 24 is even too small for a 308? ---.150+.150+.308=.608", minimum diam of a 5/8x24 thread is .562-.5742 (unless my chart is wrong) --- youd need an 11/16 thread to be safe by your calcs, right?

Maybe the chart I'm reading is wrong

 

[J E Custom]

So what are your thoughts on those 7/16" threads on the Kimber 300wm? Wouldn't that only leave about .040" at the muzzle?

You are basicly saying all 308's should be 5/8 thread or larger

YES.

With your calcs a 5/8× 24 is even too small for a 308? ---.150+.150+.308=.608", minimum diam of a 5/8x24 thread is .562-.5742 (unless my chart is wrong) --- youd need an 11/16 thread to be safe by your calcs, right?

It is just above My limit minimum of .125

Maybe the chart I'm reading is wrong

When we calculated barrel wall using the recommended wall thickness using the barrel makers MINIMUM required to flute a barrel, I got recommended numbers from them of .137 to .150 for thickness from bottom of flutes to the bore groove. being safe I used the .150 for a base line. this is the reason I excepted the .125 in some less intense cartridges.

I believe that 1/2" 24 is as small as you should get for anything under 6mm. 9/16 works up to .284. 5/8 x 24 is recommended for .308, and 11/16'' for .338 ETC.

This can be altered by going to finer threads like 32 TPI. this will reduce minor diameter buy a small amount, but it is not as common as the 24 pitch.

Then you have to look at barrel diameter at the muzzle to see if you have enough over the major diameter + .a minimum of .020 shoulder to seat the brake against.

Many rifles were not designed to except a muzzle brake because of barrel contour (Diameter at Muzzle) so a clamp on type brake should be used.

I have seen muzzle brakes installed with .060 to .070 barrel wall thickness that shot off and other that did not so when you are in this area of barrel wall thickness. barrel quality and thread quality have a major part in failure or not.

The other downside of thin barrel wall thickness is that the muzzle brake can reduce the bore size or allow it to increase, I just removed a home made brake that had reduced the bore size by .003 thousandths.

lots of smiths are more conservative than Me and don't/won't install brakes without more barrel wall thickness.

So if you keep the thickness to a minimum range of .125 to .150 and have a shoulder of a minimum of .025 to seat the brake against you will be OK.

J E CUSTOM

 

[cohunt]

Thanks that helps and kinda what I was thinking.

So your thoughts on those kimber 308w, 30-06, 300wsm and 300wm with 7/16" threads?

 

[J E Custom]

Thanks that helps and kinda what I was thinking.

So your thoughts on those kimber 308w, 30-06, 300wsm and 300wm with 7/16" threads?

I would never use a 7/16 thread tenon on anything. The 1/2"
is The lower limit and preferably larger if the bore dictates it .

If the barrel is to small at the muzzle, for a thread that meets the requirements mentioned earlier, a clamp on brake is the only option.
The older Weatherby's had super light barrels for carry and are normally not suitable for a brake.

Unfortunately some of the rifles that need a brake the most are the ones with the very light barrels and not much can be done with them except load the butt stock up with lead shot, and that normally defeats the purpose for the light rifle.

J E CUSTOM

 

[tobnpr]

If there is inadequate diameter for a shoulder at the muzzle, a custom brake with internal shoulder can be seated against the muzzle face.
No less effective at holding the brake concentric-provided both surfaces are perfectly perpendicular to the bore- than seating the rear of the brake to the barrel shoulder behind the threads.