Barrel TQ and Headspace Crush Measurements

[tobnpr]

Minimum and consistent thread crush is a result of having torque shoulders that are perfectly perpendicular to the threads. Both internal/external, receiver ring and barrel shoulder.

When this is the case, full contact occurs when the parts are brought together.
In the case of muzzle devices- I'll spin on a brake most of the way, then "wring" the brake to the tenon shoulder to check it. With no torque applied, it's very difficult (or impossible) to remove without a strap wrench.

When it's not the case- you end up with partial contact being made, with torque being required to get the rest of the surfaces to make contact. Not what you want, and this makes it impossible to consistently gauge how much crush to allow for. Usually I add .0015, and rarely does it vary by more than .0005.

Consistent with this, I find barrel torque values to be relatively insignificant. No torque wrench, 50-75 ft. lbs.
Never had one break loose.

Just my take.

 

[Jesswil61]

One thing I see not mentioned, Dave kind off hits on in his comparison of the gas gun barrel extensions where he adjusts the clearance. I my thoughts the threads only provide a tensioning mechanism for the two mating surfaces. Variations in thread clearances and lubrication applied will affect the gained tension achieved when a specific torque is used. The tighter the thread, the more resistance in the rotational direction which will result in less joint tension, whereas loose threads and some lubrication will achieve the most tensioning on the mating surfaces. Still with the variables here and relatively squared and true surfaces should not compress by an amount that would affect chamber head space, if is does maybe the headspace is the issue.

 

[jelenko]

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

How would a lot of torque even affect the cold bore shot?

With the little I know about this stuff, that sounds like 'snake oil'. I'd want to confirm that they are actually putting 500 ft-lbs on. And, how would you even do that?

 

[Dusty Stevens]

How would a lot of torque even affect the cold bore shot?

With the little I know about this stuff, that sounds like 'snake oil'. I'd want to confirm that they are actually putting 500 ft-lbs on. And, how would you even do that?

Ive seen kelbly install the panda inserts with 350lbft and its scary

 

[jelenko]

Ive seen kelbly install the panda inserts with 350lbft and its scary

So, I had to look up what an insert is for a Panda.

What is the benefit of putting a steel insert into an aluminum shell? Just weight reduction? If so, what do competitors use the extra weight for?

 

[Edd]

500 ft pounds.

the “gunsmith“ that wrote that is a moron.

Someone that is booked a year in advance selling a Model 700 action, with a Krieger barrel, and a Mcmillan stock for $7000 is not a moron.

 

[AlNyhus]

My Panda at 100 lbs/ft. -Al

 

[Dusty Stevens]

Someone that is booked a year in advance selling a Model 700 action, with a Krieger barrel, and a Mcmillan stock for $7000 is not a moron.

Jesus himself couldnt build a remington for $5000…. Alan Hall @1998

 

[Tokay444]

500 ft pounds.

the “gunsmith“ that wrote that is a moron.

Mike R from Tacops tightens his barrels to over 500ft/lbs and his rifles speak for themselves. He also has a proprietary thread that’s more like a buttress than 60°….
Edit. Wow, the article was about Tacops.

 

[natdscott]

500 ft pounds.

the “gunsmith“ that wrote that is a moron.

I don’t know what he uses, and I do doubt it is 500 lbs, but if this IS attributable to him, I can assure you that Mike Rescigno is no moron.

He has done some unconventional things to attain unconventional performance from “stock” actions such as the Remington 700. His processes are also entirely behind closed doors, so… we just have to judge him by the piles of rifles that shoot in the 0 to 1 range….

 

[wkdickinson]

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.

View attachment 1317342

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!

View attachment 1317341

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.

View attachment 1317343

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).

View attachment 1317340

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?

Not being a mechanical engineer or having any serious pedigree, I think the reason you found the headspace didn't change much between 45 and 100 lbft was "stretch". Remember, the way threads lock themselves, is by stretch the bolt. So while you may have rotated the joint another 2 deg., it likely just stretched the the tenon and did not appreciably change the relationship between the chamber and the bolt face - "headspace". My guess is if you had continued apply more torque, you would have found the same thing, the tenon would have continued to stretch, but the headspace would have stayed essentially the same.

 

[DaveTooley]

Ive seen kelbly install the panda inserts with 350lbft and its scary

I'll bet they don't back out when removing a barrel.

 

[DaveTooley]

Not being a mechanical engineer or having any serious pedigree, I think the reason you found the headspace didn't change much between 45 and 100 lbft was "stretch". Remember, the way threads lock themselves, is by stretch the bolt. So while you may have rotated the joint another 2 deg., it likely just stretched the the tenon and did not appreciably change the relationship between the chamber and the bolt face - "headspace". My guess is if you had continued apply more torque, you would have found the same thing, the tenon would have continued to stretch, but the headspace would have stayed essentially the same.

I don't think, in fact I know stretch isn't the correct word. At one time I worked in the metal building business. It takes a 3/4" drive torque wrench on the structural bolts. That's a lot of torque. If they were torqued to 80 ft/lbs the steel frame would soon be rocking and swaying like a dried out rocking chair.
Let's say you have two gage blocks and you ring them together, meaning there is no air between them. You can have confidence that the overall measurement of the combined blocks is accurate. That's the way they work. The junction between the barrel and action is similar unless influenced by other forces. With a good finish when they come together, they are together and HS won't change. Rotational change is influenced by other factors.

 

[Tokay444]

@jelenko

What is the benefit of putting a steel insert into an aluminum shell?

Not blowing one’s self up is the benefit.

 

[dnellans]

My Panda at 100 lbs/ft. -Al

That’s clearly many degrees of rotation more than I get. Can you measure a headspace difference between snap tight and 100lbs?

 

[dnellans]

I don’t know what he uses, and I do doubt it is 500 lbs, but if this IS attributable to him, I can assure you that Mike Rescigno is no moron.

He has done some unconventional things to attain unconventional performance from “stock” actions such as the Remington 700. His processes are also entirely behind closed doors, so… we just have to judge him by the piles of rifles that shoot in the 0 to 1 range….

I don’t mean to denigrate his work needlessly but I’m a continual skeptic. If there are piles of his rifles shooting 0 or 1’s out there - why are all the competitors here not using him? Extraordinary claims require extraordinary evidence.

The article I pulled that quote from couldn’t actually even recreate his accuracy guarantee… but they were sure the shooter was the issue

 

[butchlambert]

I don’t know what he uses, and I do doubt it is 500 lbs, but if this IS attributable to him, I can assure you that Mike Rescigno is no moron.

He has done some unconventional things to attain unconventional performance from “stock” actions such as the Remington 700. His processes are also entirely behind closed doors, so… we just have to judge him by the piles of rifles that shoot in the 0 to 1 range….

Don't take this wrong, but I've never heard of the guy. Who does he build for?

 

[jackieschmidt]

Someone that is booked a year in advance selling a Model 700 action, with a Krieger barrel, and a Mcmillan stock for $7000 is not a moron.

I offer my apologies to the guy I called a moron for using 500 ft/lbs of torque to install a barrel.

If he can sell it to his customers, more power to him.

 

[AlNyhus]

That’s clearly many degrees of rotation more than I get. Can you measure a headspace difference between snap tight and 100lbs?

The camera angle exaggerates it a bit. If there's even .0005 difference from firmly hand tight to torqued, I sure can't measure it. -Al

 

[Dusty Stevens]

Ive never heard of him either and there aint many circles im not involved in especially rifles that shoot zeroes and ones.
what im more interested in is how a tighter barrel shoots cold bore shots better and howcome these guns are not in the match reports with the guns that do shoot in the zeroes and ones?