Installing comp/FH . . . does amount of torque affect accuracy?

[Ned Christiansen]

Update soon.

[jsbhike]

I just received a few Armageddon Tactical crush washers, but haven't installed yet.

While waiting on those, I ordered 1/2×28 jam washers which sound like a better idea in some ways.

[Ned Christiansen]

Jam washers-- what is that? Did you mean jam nut or lock washer maybe?

So in my little bit of testing I found the JP Double Crush washer gave a very constant 15 ft lbs over about 1 1/4 turns. That's more than you need, really but certainly no harm in it. The Armageddon gave 17-20 and about 3/4 turn. I tested several of each and results were very consistent. I think either is an upgrade from standard crush washers.

I tested a few standard ones too and got different results from a previous test, a reminder that A) there is likely to be some variance in crush washers and B) these are not laboratory tests. The couple-three I tested with the JP and Armageddon products actually gave pretty decent, 20-to23-ft-lb results. Previously it was about 40 and that was enough to cause some detectable bore constriction. I think there might be two kinds of "standard" crush washers out there (or more), or they maybe just be inconsistent lot to lot, or something. I will say that the JP and Armageddon washers are for-sure purpose-built and engineered for this one application, and we know the source of them and it's not generic. That's a plus. As to economy I don 't know as I requested and received no-charge samples from both. As to the monetary value of them I will say that if they are 15% better and 4X the cost, well we won't be seeing them on production rifles but for the average guy doting on a personal gun or two and striving to improve even the little things, it's probably not that big a deal.

The thing about testing stuff is that the big revelation is usually that more testing is needed! And that your test needs testing, and that little things cause big variances that need more.... testing.

I've never seen a flash hider that had thread locker on it from the factory.... that doesn't mean much but I also have never seen a military or Colt manual say anything about either lubing or thread-locking muzzle threads. Personally I like having something lube-ish on those threads. I don't use thread locker unless it's someone's QD mount for a suppressor where it's liable to have a lot of turning force applied to it regularly.

[jsbhike]

Jam washers-- what is that? Did you mean jam nut or lock washer maybe?
.

Yes, jam nut. Had a brain to finger interface failure

[556Cliff]

Thanks for doing this little test Ned!

I've had good experiences with the Armageddon Tactical crush washers, my only concern with them is that they don't offer as much rotation as you sometimes need. So I switched to the JP double crush washers but haven't had the opportunity to try them out yet. I've had mixed results using standard crush washers though, some crush fine and I've had others that don't want to crush easily at all.

So did you have the threads lubed or dry during the test? At the end of your post you mentioned that you prefer to use lube on muzzle threads, so I'm just assuming that you probably used lube for the test?

[georgeib]

Jam washers-- what is that? Did you mean jam nut or lock washer maybe?

So in my little bit of testing I found the JP Double Crush washer gave a very constant 15 ft lbs over about 1 1/4 turns. That's more than you need, really but certainly no harm in it. The Armageddon gave 17-20 and about 3/4 turn. I tested several of each and results were very consistent. I think either is an upgrade from standard crush washers.

I tested a few standard ones too and got different results from a previous test, a reminder that A) there is likely to be some variance in crush washers and B) these are not laboratory tests. The couple-three I tested with the JP and Armageddon products actually gave pretty decent, 20-to23-ft-lb results. Previously it was about 40 and that was enough to cause some detectable bore constriction. I think there might be two kinds of "standard" crush washers out there (or more), or they maybe just be inconsistent lot to lot, or something. I will say that the JP and Armageddon washers are for-sure purpose-built and engineered for this one application, and we know the source of them and it's not generic. That's a plus. As to economy I don 't know as I requested and received no-charge samples from both. As to the monetary value of them I will say that if they are 15% better and 4X the cost, well we won't be seeing them on production rifles but for the average guy doting on a personal gun or two and striving to improve even the little things, it's probably not that big a deal.

The thing about testing stuff is that the big revelation is usually that more testing is needed! And that your test needs testing, and that little things cause big variances that need more.... testing.

I've never seen a flash hider that had thread locker on it from the factory.... that doesn't mean much but I also have never seen a military or Colt manual say anything about either lubing or thread-locking muzzle threads. Personally I like having something lube-ish on those threads. I don't use thread locker unless it's someone's QD mount for a suppressor where it's liable to have a lot of turning force applied to it regularly.

Thanks, Ned!

[T2C]

Old time Service Rifle smiths such as Compass Lake Engineering or White Oak have known for decades that excessive torque on muzzle devices negatively affected accuracy, why they usually tighten to 10-15 foot pounds and sometimes use a non permanent lock tite to prevent loosening. Suspect its also true if no washer is used, as stress is ultimately transmitted via threads to the barrel.

Mark

This is why I use a generous amount of Loctite on the threads and install a flash hider hand tight on match barrels. If the threads are cut to close tolerances, Loctite Blue works. If the thread tolerances are a little loose, I use Loctite Red.

[Ned Christiansen]

T2C, I think you nailed what is probably the very best approach. 556cliff, I should know this but I don't specifically remember lubing the threads..... but if I know me, I did :-)

I have one barrel I went up to 90 ft pounds on. It formed a ring in the barrel at the shoulder (back end of threads) which passes a .206 pin but not a .207. Ouch.

Edit: "Best approach" when you're going to leave it alone for a long period of time. I changing out stuff on an irritatingly regular basis so I don't sue any locker.

[Hohn]

This is why I use a generous amount of Loctite on the threads and install a flash hider hand tight on match barrels. If the threads are cut to close tolerances, Loctite Blue works. If the thread tolerances are a little loose, I use Loctite Red.

I think it's odd the muzzle constriction is thought to be detrimental to accuracy in the case of a threaded MD, while many fine precision rifles have barrels that feature a very slightly smaller bore near the muzzle as an accuracy enhancement. It's well known (or rather, commonly believed) in rimfire circles that such a couple-tenths constriction (detectable on air gauge) can enhance accuracy. I've seen similar comments on centerfire BR barrels as well.

It's one thing to show that high torque causes bore constriction, but quite another to show that it is detrimental to accuracy at some level of significance. It seems like the latter would be pretty hard to prove unless it was nearly catastrophic.

Also, the barrel threads don't see "torque" per se. They see the stretching effect of tension induced by the torque. But the conversion of that torque to tension depends on the friction in the threads (the so-called nut factor or "k-factor" in nerd speak). So 30lb-ft isn't a magical safety zone under all conditions. If you have super slick threads (like say you used some kind of Molykote on them) you might be able to actually yield that barrel at 30 lb-ft. And if the barrel is solvent-cleaned and has no oils on the threads, it might take 60+ lb-ft to yield the barrel.

I gained an appreciation for this relationship when I was doing some development on an exhaust manifold. I had specified a kind of dry film lube on the screws since our mfg plant doesn't do liquid anti-seize. The torque of 65Nm on an M10 should give right at 36kN of load with a k-factor of 0.18.

The screws came in from the supplier and I couldn't even get them to 50Nm before they yielded like a cheap suit. I was gobsmacked. Subsequent investigation ruled out soft screws or material issues. It turns out that the supplier couldn't get the dry film lube I specified and instead substituted something the supplier of such told them was essentially equivalent to what I specified.

When I tested the alternative coating, I found that the k factor dropped from 0.18 to 0.10. Which means it would take only about half the torque with this other coating to get the same amount of load as with the coating I originally specified.

I later learned that even the dilution rate and such of the liquid coating (before application) would affect the tested k-factor and therefore the correct torque.

So to get the proper load on that joint, I had to specify not only the coating, but the dilution as well on top of a torque spec with tolerance.


So-- before heading off thinking that 30lb ft is always safe or that 45 is always bad, it pays to know the circumstances under which those tests where performed. Loctite will give different results than will an oil, different from a moly grease, different from clean thread, different each time you use a different lube/locker/antiseize on those threads. A torque and tension relationship is only good for one set of conditions. Change those conditions, you change the relationship.

[T2C]

When I first shot a politically correct match rifle without a flash hider, there was a noticeable improvement in accuracy, especially at 600 yards. The barrels were from the same manufacturer and sold by the same vendor. Reload recipes were the same, right down to identical components, and the number of times brass was fired was the same.

I don't have scientific data, just results on target to lead me to believe that stresses induced anywhere on a barrel will cause a change in accuracy. Roughly 45 years ago, a rifle builder told me to think of a barrel as a tuning fork. Anything that changes vibration will cause changes in accuracy. I believe that excessive torque on a muzzle device adversely affects accuracy. The $40,000 question is how much torque is excessive?