Barrels unscrewing from the action.

[MikeMcCasland]

Long story here. Had a barrel with ~2,000 rounds that was shooting really well, and then all of a sudden performance took a huge nose dive. Everything on the bench remained the same; same lot of everything. My loading processes aren't at fault here.

Within ~100 rounds it went from holding inside of 10 ring vertical at 1k (so long as I didn't miss a condition) to tossing 8s (a lot of them) in conditions where it shouldn't happen.

I went through a litany of things to troubleshoot the barrel. I tried different depths (.012 ahead of the curerent node and working back .020, in .003 increments), and just couldn't get it to shoot that well. I thought I had it sorted with a small harmonics change where it did start shooting a little tighter, but still wasn't all that great.

Further confusing things is while all this is going on, the rifle is putting up great numbers on the choronograph. Velocities hadn't fallen off since new, and it put up an ES of 22 FPS & an SD of ~5 FPS across 42 rounds.

All of the above was driving me crazy. I couldn't tell if it was shot out (~2,000 rounds), or just fell way out of tune somehow.

Last night I decided to just cut my losses and pull the barrel. That's when I found something interesting. I went to pull it off, and it required almost zero force to back the barrel off the action; I'd guess ~5ft lbs? I barely touched the thing. (and no, I wasn't the one who torqued it on the action).

So it's not got me thinking that perhaps it was slowly starting to back off the action, and that's why I was seeing such bad on-target performance, couldn't tune it with depth, yet was still seeing really good chronograph numbers.

My question to guys that have had barrels back off: How did the guns behave before you identified your problem as a loose barrel? Was it a sudden and immediate loss of performance, or did it linger for a while?

 

[F Class John]

I wish I could say it's never happened to me but it has, and what happened to you WAS EXACTLY WHAT HAPPENED TO ME on every detail. Even the part about figuring the barrel was bad and swapping it out only to find I didn't need any wrench to take it off, it just spun right off. I'll be honest that before it happened I had a habit of using a standard socket bar and just snugging the barrels down pretty hard by hand and I swore that kind of thing would never happen to me. Ever since that event I now use a proper torque wrench and hit 80FT/lbs every time and it's never happened since.

 

[msalm]

Only one I’ve seen do that was a Bat action with the recessed threads. Previous gunsmith didn’t add a 45% chamfer to the barrel shoulder and only the edge of the shoulder contacted the radius at the end of the action counterbore. Other than that one instance I have not seen a bbl come loose, although I do torque barrels on pretty good myself.

 

[jelenko]

My experience was on an AR15 [with free float forend].
Mine took a nose dive in a match - so the big difference in accuracy came on quickly.
It may have been loosing some accuracy before the match - but couldn't have been much or it wouldn't have gone to the match.
In my case, the barrel had actually come loose.

 

[MikeMcCasland]

I wish I could say it's never happened to me but it has, and what happened to you WAS EXACTLY WHAT HAPPENED TO ME on every detail. Even the part about figuring the barrel was bad and swapping it out only to find I didn't need any wrench to take it off, it just spun right off. I'll be honest that before it happened I had a habit of using a standard socket bar and just snugging the barrels down pretty hard by hand and I swore that kind of thing would never happen to me. Ever since that event I now use a proper torque wrench and hit 80FT/lbs every time and it's never happened since.

Very interesting, and tentatively validating. It makes me somewhat optimistic I can just screw this thing back on (tight) and keep shooting it. It was shooting well, and then just took a nose dive. Unfortunately I won't be able to mess with it for another 3-4 weeks with all the matches scheduled.

I do want to clarify, I never tried to unscrew mine by hand. I don't know if it would have unscrewed that way, and the engraving was still indexed correctly. My wrench is a t-handled one, so no way to quantify the force. That said, it backed off with VERY little effort; I'd guess no more than 5ft-10ft lbs.

Thank God for having identical comp rifles lol.

 

[F Class John]

...I do want to clarify, I never tried to unscrew mine by hand. I don't know if it would have unscrewed that way, and the engraving was still indexed correctly...

So when I did mine I chucked up my barrel in the vice, put my action wrench in and then attached my handle. Normally I have to apply a decent amount of force to pop it lose but in this case without even thinking, I just started spinning the action off and after about half a rotation it occurred to me that something was a astray....that was my experience. Hope it helps.

Oh, once I torqued it back it shot great again. But as always, YMMV.

 

[MikeMcCasland]

So when I did mine I chucked up my barrel in the vice, put my action wrench in and then attached my handle. Normally I have to apply a decent amount of force to pop it lose but in this case without even thinking, I just started spinning the action off and after about half a rotation it occurred to me that something was a astray....that was my experience. Hope it helps.

Oh, once I torqued it back it shot great again. But as always, YMMV.

Ok...yeah. Exact same thing here. There was no 'pop' to it. It just started spinning after very light resistance. Maybe two fingers worth of pressure.

Felt kinda like when I start torquing scope ring screws at 10 inch lbs.

 

[BP1]

Well while you’re at it go ahead and check your other rifle to..just incase Mike ..also check all your screws on your scope and barreled action.

 

[MikeMcCasland]

Well while you’re at it go ahead and check your other rifle to..just incase Mike ..also check all your screws on your scope and barreled action.

Already ahead of you. That rifle is ready to hurt some feelings this weekend.

 

[tmwinds]

If you need to use excessive force to remove a muzzle brake always retorque the barrel afterwards. I loosened a barrel when I had to use a screwdriver and mallet to remove a radial brake. I suspect the sharp jolt from the mallet caused it to loosen. Fortunately I saw the barrel move and retorqued it.

 

[Dusty Stevens]

Did your fired brass not measure different?

 

[MikeMcCasland]

Did your fired brass not measure different?

Same so far as I can tell.

My notes say fired brass is 1.619 coming out of that gun. I measured 1.6185-1.6195 across the last 10 I fired. I don't think it unscrewed. I think was a couple of shots away from doing so.

 

[Jdne5b]

My last 308 barrel died inside of 200 rounds at 1800ish total rounds. Well.. I think it died. Too many matches coming up and primer availability made me take it off and not mess with it any more.

The last 3 matches:
596
587 (with a 200 low x count in there)
578. I came home from this match and tested ammo. Nothing was under .8moa at 200yds and it was tossing 8s like a salad this last match.

So.. you may have a toast barrel. But I'll watch for an update when you have time to mess with it.

 

[BP1]

Already ahead of you. That rifle is ready to hurt some feelings this weekend.

Good luck with that

 

[INTJ]

Earlier this year I shot the LR BR rifle I had just built. 300 WSM, with a brake, 600 yds. I shot second relay. I did shoot a 1.5" group the second round of Light Gun. After that everything went to pot.

When I was cleaning the rifle after the match I started to unscrew the brake and the barrel unscrewed.

I tightened the barrel to 60 ft lbs and tested the load. I then shot one more match and the rifle did much better.

 

[dnellans]

Some math facts (someone check my math!) to help diagnose.

18TPI barrel tenon moves .056 inches in a full 360 degree rotation, that means even if it clocked loose by 1 degree you're at .00016 difference in headspace, 10 degrees (which you'd probably notice) is still only .0016.
thats probably within a lot of guys brass margin of error, so its probably not a headspace issue...

So if its not a headspace issue, is it an thread quality issue? hand tighten your barrel but don't snap it tight. now back it off ~10 degrees and stick a .001 shim between the receiver and the barrel. remove the shim and try and wiggle the barrel, can you? you get a 14x (28 inch barrel to .5 inch shoulder) magnification of the deflection at the barrel shoulder in terms of muzzle deflection. if the threads are loose and the shoulder lost contact that can result in a lot more wobble at the muzzle than you might think, now extend that same angular measurement out to 100 yards or beyond.

Now that said - if your barrel was 5lbs or 10lbs tight and the shoulder is dead nuts square to the receiver, it won't deflect at 5lbs, 45lbs, or 100lbs of torque... i have never been able to measure "crush fit" when torquing a barrel like you hear some guys talk about. 10 degrees of rotation is noticeable while you're staring at it and holding a torque wrench.

just some food for thought

 

[INTJ]

Some math facts (someone check my math!) to help diagnose.

18TPI barrel tenon moves .056 inches in a full 360 degree rotation, that means even if it clocked loose by 1 degree you're at .00016 difference in headspace, 10 degrees (which you'd probably notice) is still only .0016.
thats probably within a lot of guys brass margin of error, so its probably not a headspace issue...

So if its not a headspace issue, is it an thread quality issue? hand tighten your barrel but don't snap it tight. now back it off ~10 degrees and stick a .001 shim between the receiver and the barrel. remove the shim and try and wiggle the barrel, can you? you get a 14x (28 inch barrel to .5 inch shoulder) magnification of the deflection at the barrel shoulder in terms of muzzle deflection. if the threads are loose and the shoulder lost contact that can result in a lot more wobble at the muzzle than you might think, now extend that same angular measurement out to 100 yards or beyond.

Now that said - if your barrel was 5lbs or 10lbs tight and the shoulder is dead nuts square to the receiver, it won't deflect at 5lbs, 45lbs, or 100lbs of torque... i have never been able to measure "crush fit" when torquing a barrel like you hear some guys talk about. 10 degrees of rotation is noticeable while you're staring at it and holding a torque wrench.

just some food for thought

Well, my barrel was only hand tight and brass shoulders lost .001" when I torqued the barrel properly.

I have also noticed that when I have the action hand tight on the lathe, it seems to lose another .001" of headspace when torqued in the barrel vise.

 

[snert]

Long story here. Had a barrel with ~2,000 rounds that was shooting really well, and then all of a sudden performance took a huge nose dive. Everything on the bench remained the same; same lot of everything. My loading processes aren't at fault here.

Within ~100 rounds it went from holding inside of 10 ring vertical at 1k (so long as I didn't miss a condition) to tossing 8s (a lot of them) in conditions where it shouldn't happen.

I went through a litany of things to troubleshoot the barrel. I tried different depths (.012 ahead of the curerent node and working back .020, in .003 increments), and just couldn't get it to shoot that well. I thought I had it sorted with a small harmonics change where it did start shooting a little tighter, but still wasn't all that great.

Further confusing things is while all this is going on, the rifle is putting up great numbers on the choronograph. Velocities hadn't fallen off since new, and it put up an ES of 22 FPS & an SD of ~5 FPS across 42 rounds.

All of the above was driving me crazy. I couldn't tell if it was shot out (~2,000 rounds), or just fell way out of tune somehow.

Last night I decided to just cut my losses and pull the barrel. That's when I found something interesting. I went to pull it off, and it required almost zero force to back the barrel off the action; I'd guess ~5ft lbs? I barely touched the thing. (and no, I wasn't the one who torqued it on the action).

So it's not got me thinking that perhaps it was slowly starting to back off the action, and that's why I was seeing such bad on-target performance, couldn't tune it with depth, yet was still seeing really good chronograph numbers.

My question to guys that have had barrels back off: How did the guns behave before you identified your problem as a loose barrel? Was it a sudden and immediate loss of performance, or did it linger for a while?

I had a 22 PPC that apparently was only hand tight on a custom action. It grouped great. I was shooting groundhogs and one day it went from awesome to awful. Hits went WAY low.

I thought I whacked the scope.

Went to the range and it was grouping WAY low. I adjusted the scope and it was fine...till I picked up the gun by the barrel and felt it move. I realized it was loose. I hand tightened it. It returned to original point of aim, so I re-adjusted scope. Did I mention it was like 10 inches low at 100.

I took it to the smith who put it on tight with a barrel wrench. Then all my brass was too tight. GRRRRRRR.

i preferred it hand tight!

 

[carlsbad]

While you're doing your math add a force balance. The threads need to resist the recoil pulling the rifle back. The heavier the barrel the more inertia. It's hard to measure g's but impact loads would result in infinite acceleration (and break things) if movement was instantaneous. Clearly it isn't but it is pretty darn fast. So if the action accelerates away from the barrel fast enough cause a force pulling the barrel back forward greater than the clamping force applied by the threads, then the shoulder joint will momentarily open. on a bolted joint, the stretch of the fastener is designed to be more than any dynamic force on the joint, thus keeping the joint from opening. Because the barrel tenon has to be thick to contain the pressure of the cartridge, the torque does very little to stretch the tenon and more of the load is taken up in elastic deformation of the threads (this is fine so long as it remains elastic). Good threads with good contact area lock up in a degree or 2 and then move very little while tightening. This is very hard to calculate without FEA.

If you do want to calculate the clamping force on the shoulder, then the formula is T=.2Fd. T is Torque in Ft lbs. F is force in lbs and d is diameter of the thread in Feet (not inches). .2 if the "nut factor" associated with the threads. I like to use .2.

So while we're not going to stretch the tenon we can see that 35 fl-lbs of torque will give about 2000 lbs of clamping force. 5 ftlbs will only give a couple hundred and I'd say that could be overcome by the impact load of firing the rifle.

--Jerry

 

[std7mag]

Proof Research has a video on Youtube to show how strong their carbon wrapped barrels are.
Barrel was hand tight. Shot 3 rounds, removed & used to bludgeon a cinderblock to pieces.
Then hand tightened back onto the action &shot another 3 rounds with virtualy no POI shift.