There have been a couple of threads in the last year where some people have said they see as much as .003 headspace crush when torquing barrels and people are using anything from snap-tight, hand-tight, 45, 65, 100, 150 ft/lbs when putting barrels on.
I just read an article this morning where a custom gun shop says they use 500 (that is not a typo) ft/lbs on standard rem 700 actions because it reduces cold bore fliers
Ever the scientist I decided to actually do some measuring last night while getting a gun ready for an upcoming match and thought I'd write it up so someone in the future can stumble upon it.
Action: Bordan Mountaineer - 16 TPI (important later)
Barrel: Krieger stainless
Gunsmith: Me
I normally use a tq wrench and set barrels at 45ft lbs for consistency and have never noticed any measurable headspace change (sub .001) between a "snap tight" action fit on the lathe to check against a go guage and having torqued it at 45 ft lbs.
What the math says (hopefully I get this right on first try):
The math on a 16TPI barrel is that 1 degree of rotation (1/16TPI)/360 = .000174. So just shy of 2 tenths of headspace.
5.76 degrees of rotation is thus required to get to even .001 of headspace change, between what I'll call "snap-tight" and whatever your final torque value is.
Its a little hard to measure the angle of rotation on a barrel accurately to the degree without marking and aligning the end of the barrel, but you can make a few coarse grained measurements that get you close. For example: A 1.25 diam barrel at the shoulder has a 3.926 inch circumference. So if you mark a line on the barrel, it needs to travel (3.926/360) .0109 (lets call it .011) inches at the surface to have moved 1 degree. Maybe thats a little small to see and measure with calipers, but 6 degrees of change would be ~.066 of travel, which you can pretty easily see with the naked eye. Getting to .003 of headspace crush would be 3 times that or .200 of travel, which is a long ways for that line to move!
So with that in hand - onto the experiments:
Step 1: Snap tight the action (lightly lubed with red moly grease on threads and shoulder). Note that the action closes with friction on the go-guage (I looked back at notes and this was what it did when barrel was on the lathe). It will not close as all with a .001 shim behind it. Made a sharpie mark on action and barrel.
Step 2: Actually crank the action on with my hands. I wouldn't do this in the lathe but in a barrel vice, no problem. Its a little hard to tell in the photo but the line did in fact move a little bit. How much? Very hard to measure, but the go guage that had friction still closes with perceptibly the same amount of friction, so it can't be that much. Remember, this barrel wouldn't close at snap-tight on go+.001 shim, so if we tighten headspace by a full thou, the bolt shouldn't close.
Step 2: Put the torqe wrench on it and give it 45 ftlbs. Woah - that line has moved a lot! Ok not a lot, but at least a little bit, its now obviously noticable. However - the go guage still closes with same perceptible friction!
Step 3: Give it 65 ftlbs. Looking back and forth between 45 and 65 you can see it did in fact move a little bit more, but less than the move from hand tight to 45lbs. Surely it won't close on the go guage now right? Wrong - still closes with same amount of friction feel on the bolt.
Step 4: Crank it up to 100 ftlbs. I do think it moved yet a little bit more, but its really hard to tell in the picture and it also slipped in the barrel vice because i don't normally crank them this hard. Retightened the vice, checked it, didn't move any more. So while it did move - the difference back to 65lbs is now smaller than the diff of 45->65, which is smaller than hand tight to 45lb. I mean look at how much that moved, clearly we've crushed the headspace right? Well - it still closes on the go guage, but, this time I can notice more friction, and its tight enough that if it was on the lathe I probably would go in a bit deeper. So now think back, that just snap-tight, it would close on the go and wouldn't close, at all, on a .001 shim.
Just for kicks I tried to align calipers to the middle of those lines (dicey proposition) just so see how far apart they seem to be. this is a linear measurement, not on the surface of the curve, but the measurement was about .024 best I could tell. That implies about 2 degrees of movement from snap tight to 100ftlbs and a total headspace crush happening of .00035. thats 3 tenths, not 3 thou, and matches getting a little more bolt feel on close but not getting to .001 where the bolt wont close at all (like the shim).
Step 5: Pop it loose and snap tight by hand one more time just to check. Yup - friction close on the go and won't close at all on .001 shim. I then did it all one more time just to see if it was repeatable and only once at 100 ftlbs did I feel any noticable close difference on the go, but it closed never the less.
My conclusions:
A snap fit on the lather is plenty accurate to be able to measure final headspace when torqued. I was a little surprised that it continued to move beyond my normal 45 ftlbs, but its also clearly diminishing movement vs tq at anything over hand tight.
I don't have the mechanical background (or motivation) to figure out how much threads might stretch at 45lbs vs 100ftlb and if that could matter. Maybe that extra tq would micro improve the chamber alignment to the bolt face of the gun? I've certainly never had a gun loosen up at 45lbs however.
If I went up to 200lbs, I suspect that I probably could have gotten the bolt to not close on the go, but thats a god-aweful amount of tq to put on the action and I'd worry about galling the threads of action face, so I didn't want to do it.
I'll go as far as saying that if you are seeing anything > .001 headspace crush (let alone .002-3) when mounting a barrel, something about your action or barrel job is not up to the quality most of us would like to see on our guns.
For other smiths who might eventually read this - how much do you TQ and why?
I just read an article this morning where a custom gun shop says they use 500 (that is not a typo) ft/lbs on standard rem 700 actions because it reduces cold bore fliers
Ever the scientist I decided to actually do some measuring last night while getting a gun ready for an upcoming match and thought I'd write it up so someone in the future can stumble upon it.
Action: Bordan Mountaineer - 16 TPI (important later)
Barrel: Krieger stainless
Gunsmith: Me
I normally use a tq wrench and set barrels at 45ft lbs for consistency and have never noticed any measurable headspace change (sub .001) between a "snap tight" action fit on the lathe to check against a go guage and having torqued it at 45 ft lbs.
What the math says (hopefully I get this right on first try):
The math on a 16TPI barrel is that 1 degree of rotation (1/16TPI)/360 = .000174. So just shy of 2 tenths of headspace.
5.76 degrees of rotation is thus required to get to even .001 of headspace change, between what I'll call "snap-tight" and whatever your final torque value is.
Its a little hard to measure the angle of rotation on a barrel accurately to the degree without marking and aligning the end of the barrel, but you can make a few coarse grained measurements that get you close. For example: A 1.25 diam barrel at the shoulder has a 3.926 inch circumference. So if you mark a line on the barrel, it needs to travel (3.926/360) .0109 (lets call it .011) inches at the surface to have moved 1 degree. Maybe thats a little small to see and measure with calipers, but 6 degrees of change would be ~.066 of travel, which you can pretty easily see with the naked eye. Getting to .003 of headspace crush would be 3 times that or .200 of travel, which is a long ways for that line to move!
So with that in hand - onto the experiments:
Step 1: Snap tight the action (lightly lubed with red moly grease on threads and shoulder). Note that the action closes with friction on the go-guage (I looked back at notes and this was what it did when barrel was on the lathe). It will not close as all with a .001 shim behind it. Made a sharpie mark on action and barrel.
Step 2: Actually crank the action on with my hands. I wouldn't do this in the lathe but in a barrel vice, no problem. Its a little hard to tell in the photo but the line did in fact move a little bit. How much? Very hard to measure, but the go guage that had friction still closes with perceptibly the same amount of friction, so it can't be that much. Remember, this barrel wouldn't close at snap-tight on go+.001 shim, so if we tighten headspace by a full thou, the bolt shouldn't close.
Step 2: Put the torqe wrench on it and give it 45 ftlbs. Woah - that line has moved a lot! Ok not a lot, but at least a little bit, its now obviously noticable. However - the go guage still closes with same perceptible friction!
Step 3: Give it 65 ftlbs. Looking back and forth between 45 and 65 you can see it did in fact move a little bit more, but less than the move from hand tight to 45lbs. Surely it won't close on the go guage now right? Wrong - still closes with same amount of friction feel on the bolt.
Step 4: Crank it up to 100 ftlbs. I do think it moved yet a little bit more, but its really hard to tell in the picture and it also slipped in the barrel vice because i don't normally crank them this hard. Retightened the vice, checked it, didn't move any more. So while it did move - the difference back to 65lbs is now smaller than the diff of 45->65, which is smaller than hand tight to 45lb. I mean look at how much that moved, clearly we've crushed the headspace right? Well - it still closes on the go guage, but, this time I can notice more friction, and its tight enough that if it was on the lathe I probably would go in a bit deeper. So now think back, that just snap-tight, it would close on the go and wouldn't close, at all, on a .001 shim.
Just for kicks I tried to align calipers to the middle of those lines (dicey proposition) just so see how far apart they seem to be. this is a linear measurement, not on the surface of the curve, but the measurement was about .024 best I could tell. That implies about 2 degrees of movement from snap tight to 100ftlbs and a total headspace crush happening of .00035. thats 3 tenths, not 3 thou, and matches getting a little more bolt feel on close but not getting to .001 where the bolt wont close at all (like the shim).
Step 5: Pop it loose and snap tight by hand one more time just to check. Yup - friction close on the go and won't close at all on .001 shim. I then did it all one more time just to see if it was repeatable and only once at 100 ftlbs did I feel any noticable close difference on the go, but it closed never the less.
My conclusions:
A snap fit on the lather is plenty accurate to be able to measure final headspace when torqued. I was a little surprised that it continued to move beyond my normal 45 ftlbs, but its also clearly diminishing movement vs tq at anything over hand tight.
I don't have the mechanical background (or motivation) to figure out how much threads might stretch at 45lbs vs 100ftlb and if that could matter. Maybe that extra tq would micro improve the chamber alignment to the bolt face of the gun? I've certainly never had a gun loosen up at 45lbs however.
If I went up to 200lbs, I suspect that I probably could have gotten the bolt to not close on the go, but thats a god-aweful amount of tq to put on the action and I'd worry about galling the threads of action face, so I didn't want to do it.
I'll go as far as saying that if you are seeing anything > .001 headspace crush (let alone .002-3) when mounting a barrel, something about your action or barrel job is not up to the quality most of us would like to see on our guns.
For other smiths who might eventually read this - how much do you TQ and why?
