Muzzle thread torque constricting bore diameter

[VA_XTC_Shooter]

If a muzzle device is torqued too much, the bore diameter will constrict. I've seen reports that torque over 20 ft lbs on a 1/2-28 muzzle thread can constrict the bore diameter.
Here a guy did a pretty basic video showing the bore constriction.

I am curious what torque would be required to constrict the same .223 bore if the muzzle threads were 5/8-24.

Any of you smart guys know how to calculate that? I am sure there is some engineering formula out there to calculate this, but I am not an engineer and don't really know the terminology to begin searching for such a formula.

 

[gunsandgunsmithing]

Good video. I tell people, whenever possible, clean the threads up, put a drop of blue or even purple locktite on and then just snug it up. Let the locktite do the rest. I know there are examples where this isn't ideal either, mostly on machine guns with suppressors. Ya'll are on your own. Sorry. Lol!

 

[El Matador]

I don't have a high tech formula but I did run a quick calculation. The 5/8-24 thread will give you about 2.2 times as much steel compared to the 1/2-28 when looking at a cross section. That's based on the pitch diameter of the threads, not the minor diameter (I think the threads themselves add a little strength). Then I multiplied by 28/24 because the finer pitch would tend to multiply torque more than a coarse pitch. Finally the larger diameter thread would require more leverage/mechanical advantage to achieve the same clamping force, by a factor of 1.25. The result is about 3.1x, so that 20 ft lbs would roughly equate to 60 ft lbs on the larger thread.

 

[VA_XTC_Shooter]

I don't have a high tech formula but I did run a quick calculation. The 5/8-24 thread will give you about 2.2 times as much steel compared to the 1/2-28 when looking at a cross section. That's based on the pitch diameter of the threads, not the minor diameter (I think the threads themselves add a little strength). Then I multiplied by 28/24 because the finer pitch would tend to multiply torque more than a coarse pitch. Finally the larger diameter thread would require more leverage/mechanical advantage to achieve the same clamping force, by a factor of 1.25. The result is about 3.1x, so that 20 ft lbs would roughly equate to 60 ft lbs on the larger thread.

and yet not reduce the bore diameter like on on the 1/2-28, correct?
Part of what I am getting at is, if 1/2-28 can have issues with torque, why not just use 5/8-24?

The calc/formula request was to be able to estimate the torque needed to start constricting bore with other muzzle threads. I have a 9mm PCC that uses 1/2-36 for example.

 

[slowbalt]

So I choose correctly going with 3/4-24?

 

[Carl S]

If the threads are cut to spec and the two mating surfaces-shoulders are cut square you don't need much torque to hold them together.no need to distort or induce stress into anything.

 

[gunsandgunsmithing]

If the threads are cut to spec and the two mating surfaces-shoulders are cut square you don't need much torque to hold them together.no need to distort or induce stress into anything.

Right, and let a drop of loctite do it from there. Any...blue or even the weaker purple.

 

[Will Henry]

I think the pitch angle on a 12x28 and a 5/8x24 are about the same. Not that it's important at all. WH

 

[WSnyder]

I know there are examples where this isn't ideal either, mostly on machine guns with suppressors.

That’s what you use Rocksett for

 

[gunsandgunsmithing]

That’s what you use Rocksett for

That's why I said what I did. No interest in knowing how to keep a machine gun quiet. If it comes to that, we're in trouble, any way ya look at it. They'll hear me but I hope to get a few off before it matters. Yes, I'm familiar with it. I really wish they had not made it blue...fwiw. I've seen it cause problems for others.

 

[VA_XTC_Shooter]

Man, I was really hoping there would be an engineer or two on here that could help me out.
So far I have found info on typical bolt/nut fastener interactions, but not threaded cylinders just yet.

I am leaning toward a function of Poisson's Ratio and Hoo's Law. Basically we need to look at the tensile strength of the threaded portion of the barrel. Then determine the stress and strain applied to the barrel as we tighten the muzzle device. The barrel will "stretch" with this tightening as it is elastic. There is a range of teh elasticity of the steel before it fails and becomes plastic. In that range of elasticity, Poissons ratio comes into play and the inner diameter decreases. In theory, because the elasticity slope is linear, should be able to calculate X torque = Y decrease in diameter.

 

[Will Henry]

Sounds like it might slow down the twist rate too! WH

 

[butchlambert]

A thread on barrel torque and bore measuring smaller.

The TEST (barrel torque vs various outcomes)

As we all know, I have suggested 300 - 350 in-lbs torque on all the barrels used on Holeshot actions. A number of years ago I calculated the force needed to keep the shoulder of the barrel on the action during firing. That comprised of chamber pressure pushing forward and barrel whip in the...

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