Threaded Muzzle, Calfee

Pacecil

How sensitive is the human arm?
If you ever get the chance to slug a 1.450 barrel in a barrel block with 8 1/4-20 capscrews holding the top on even a 5th grader can feel were the barrel block is located.
If your getting a 0.000091 with a standard barrel and 10,000 pounds of pressure imagine how 8 small capscrews affect a 1.450 barrel.

I think Mike Marcelli posted earlier that just changing the torque setting on a barrel block affected his accuracy.How much of a change is required?
Lynn
 
Lynn said:
How sensitive is the human arm?
If you ever get the chance to slug a 1.450 barrel in a barrel block with 8 1/4-20 capscrews holding the top on even a 5th grader can feel were the barrel block is located.
If your getting a 0.000091 with a standard barrel and 10,000 pounds of pressure imagine how 8 small capscrews affect a 1.450 barrel.

I think Mike Marcelli posted earlier that just changing the torque setting on a barrel block affected his accuracy.How much of a change is required?
Lynn
Click to expand...

Lynn:

Please don't try to use my statements to support your theories. I accept that the numbers don't lie. That means that there was probably another reason why the particular barreled action I referenced in the other post did not shoot as well with the screws torqued to 30 in lbs than it did with 20 in lbs, for example the barrel had a flaw or was not perfectly straight, yada yada. Heck, maybe I scrunched something between the barrel and sleeve that was able to work itself out when I loosened the screws.

I don't have a set of bushings to push through a barrel, so I don't know if what I thought I "felt" was real or perceived. But, I can tell you with some degree of confidence I cannot "feel" a .0003" difference, let alone a .000091" difference and given the forces involved, and the fact that this was a static system, there was probably less than 1 millionth of an inch distortion in my barrel.

Take care.
 
Mike Marcelli

Mike I wasn't using your statement to support any theory real or percieved.I asked a question.
Before you go getting all dramatic and post back contact R.W.Hart and ask them what affect a barrel block has on on a barrel and when was it discovered.I will guess that you will be told that inner tubes used to be put between the barrel and the barrel block to lessen the distortion.This is all old news that has been around for 20 plus years but if it appears on the shortrange forum it is something new and unheard of.
Now before you become a drama queen why don't you simply slug a barrel then re-torque your barrel block and see if I'm lying to you? If you yourself test it and I'm found to be lying I'll put lipstick on a pig and kiss it.Yes I'll even take a picture for proof.
Lynn
 
Mike Marcelli asked, I just want to clarify where the "growth" you calculated would be occurring. Thanks

It would be at the muzzle and fall off as you move back toward the receiver.

Jackie asked, Krieger says that the cut rifled process they employ induces no ID stress in the barrel. In your opinion, how much does the bore of a typical Krieger change when I turn .010 off the OD for 2 inches length, and then put a 36 tpi on that turned spot??........jackie

If Krieger says "no stress" then there would be no change in i.d. as you machine the od or thread it. You are a machinist - do you think you can bore and rifle a steel bar and not leave stress in the machined surface? Stress relieving would remove some, but not all, the stresses left after machining. To leave no stress you would have go to temperature (annealing) that removed all hardness, and probably also leave the barrel very distorted. If when YOU machine and thread the o. d., you leave compressive stress, then the i. d. will contract slightly.

Lynn said, If your getting a 0.000091 with a standard barrel and 10,000 pounds of pressure imagine how 8 small capscrews affect a 1.450 barrel.
I think Mike Marcelli posted earlier that just changing the torque setting on a barrel block affected his accuracy.How much of a change is required?

That 10000 psi was applied to the bore. If you apply it to the o. d. you get even less change. The pressure under a collar, or block, you apply with cap screws will vary with size and torque but typically might be from 5000 psi up to 15000 psi. This pressure, as I and Varmint Al pointed out in another post, will contract the bore about .000006. This amount of compression applied to a lead slug would produce a few ounces, possibly a 1/4 pound, of variation in sliding force, so I think a person might be able to feel this.
 
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Pacecil

Thank you for that answer.It sure doesn't appear to be that much distortion but yes you can definitely feel it.I know in talking with Hart that he felt the barrel blocks were crushing the bores and that is why the inner tube wrapping was applied.The barrel wouldn't slip like it would with less tension and metal on metal.
How much distortion in your opinion affects accuracy?
Lynn
 
PACecil:

5000 psi seems to be an awfully high force to result from a 30 in lbs of torque?

Its been almost 30 years since I've done any of these types of calcs, but I came up with the following:

The volume of the effected part of the barrel:

V= pi R2 L = 3.1415 X ((1.450 in - .243 in))sq X 8" = 9.15 in3

30 in lbs / 9.15 in3 = 3.28 psi

Where have I mucked it up?
 
Lynn asked, How much distortion in your opinion affects accuracy?

Everyone seems to feel a restriction at muzzle may help accuracy. Probably of the order we're talking about here it just may. My guess is a restriction of anything less than .0002 anywhere along the barrel wouldn't affect accuracy. I arrive at this mostly because this is what I think barrel makers claim they hold their barrels to.

Mike Marcelli asked, Where have I mucked it up? (calculation of pressure under collar)

Assume you are pulling a 1/2" wide split collar around a .90 barrel with two screws that have .02 sq in area (roughly a 3/16 screw) under the threads. The screws might be torqued to bring stress in the screws to 60000 psi. This means each screw will have .02 x 60000 = 1200# tension in it. The total load pulling each half of the collar together will be 2400#. This load will be distributed over the projected area of the collar, which equals .90 x .5 = .45 sq inches. The pressure under the collar is then equal to 2400/.45 = 5333 psi.
 
pacecil said:
Mike Marcelli asked, Where have I mucked it up? (calculation of pressure under collar)

Assume you are pulling a 1/2" wide split collar around a .90 barrel with two screws that have .02 sq in area (roughly a 3/16 screw) under the threads. The screws might be torqued to bring stress in the screws to 60000 psi. This means each screw will have .02 x 60000 = 1200# tension in it. The total load pulling each half of the collar together will be 2400#. This load will be distributed over the projected area of the collar, which equals .90 x .5 = .45 sq inches. The pressure under the collar is then equal to 2400/.45 = 5333 psi.
Click to expand...

The bold part above is what I'm having a difficult time calculating. (The area may also be difficult to calculate being this is a v-block). The torque on each of the 8 screws is 30 in lbs, +/- 1 in lb. That's not much force, but the gun did shoot better when re-torqued to 20 in lbs. Now I've cranked the hell out of action v block screws on rail guns before I knew "Better" and the gun shot fine. Before this particular barrel, after I started torquing barrels, I torqued to 40-45 in lbs and they shot fine. So I'm not 100% convinced I had a constriction in this barrel that was causing problems. However, I would like to know what force I may have been placing on the barrel to perhaps be able to rule it out in my mind.

If you want to shoot me an e-mail, feel free. ppcbrshooter@hotmail.com
 
Surely the barrel used by Bill for this test should have been of typical benchrest quality. Otherwise, how can any conclusion can be drawn ?

Incidentally, Anschutz sporting rimfire rifles, like the 1517 17hmr and 1417 22lr, are factory threaded with the muzzle counter bored to the depth of the threaded section, so that the crown is effectively countersunk from the muzzle. But maybe this is not relevant due to different barrel/thread thickness. Just my 2 cents worth.
 
Mike Marcelli said, The bold part above is what I'm having a difficult time calculating. (The area may also be difficult to calculate being this is a v-block). The torque on each of the 8 screws is 30 in lbs, +/- 1 in lb.

A V-block would change the pressure exerted on the barrel. It would be simply four forces similar to what a chuck applies to a round part. Like Varmint Al showed in his post. As to converting torque to load in screws you can use an approximate relationship of T=.2DF. Applying this to my collar where I used 3/16 screws, you would have a torque T of 45 in lb applied to a D of .187 will give a force F equal to 1203 #. To convert this to stress you divide by the area at the root of the threads, which I gave as roughly .02 sq in. so: 1203/.02=60150 psi.
The link given by Vibe gives a way to get the area of the root of the threads, that is , the stress area.
 
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