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After years, I figured out why bases get loose

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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: August/21/2015 at 22:10
Clark View Drop Down
Optics Apprentice
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Joined: February/13/2004
Location: Near Seattle
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Points: 120
I have hundreds of rifles and when I take another beater rifle I just rebarrelled to the range, half the time I get lousy groups, it is loose scope bases.
I take tools to the range and help other shooters, and half the time they get bad groups, it is loose bases.

Typically two Weaver scope bases and each base has two 6-48 screws.
Grade 5 bolts are good for 127 ksi in tension
6-48 screws have a 0.12" root [minor diameter]
Area = pi r squared = 3.140(0.120/2)^2 = 0.0112 sq in
Tension = area time stress = .0113 sq in 127 kpsi = 1456 pounds tension per screw
There are 4 screws = 5745 pounds potential clamping force.
The screws are only loaded up typically to 50% of rating = 2873 pounds clamping.

If I look in quickload at the 6.5-06 I just built with a 142 gr Nos Bal tip Long range bullet accelerates from 500 fps to 2000 fps in 0.4 ms.
An 8 pound rifle in recoil reaction would then accelerate in proportion from 1.27 fps to 5.07 fps in 0.4ms
This is a peak acceleration of 9500 ft/sec sec
If a 2 pound scope were attached to a 6 pound barrelled action and stock they would act as an 8 pound single mass.
The mass of the 2 pound scope is = [weight]/[gravity] =[2]/[32 f/ss] = 0.0625 slugs
Force = mass acceleration = [.0625 slugs ][ 9500 ft/sec sec]= 594 pounds of force pushing back on the scope in recoil

The Mausers I just built is made of steel, as are the modified S54 and S46 bases.
Looking up the coefficient of static friction of steel on steel:
https://en.wikipedia.org/wiki/Friction 
is 0.74 ~ 0.80 for dry and clean steel
Averaging the two would be 0.77
The static friction force threshold of slipping = [coefficient of friction clean steel][clamping force] = [.77][2873 pounds= 2212 pounds.
Because the static friction 2212 pounds is greater than recoil reaction on the scope mass 594 pounds, the scope should not slip.

But the coefficient of friction for lubricated steel on steel is 0.16
The static friction force threshold of slipping = [coefficient of friction lubricated steel][clamping force] = [.16][2873 pounds= 459 pounds.
Because the static friction 459 pounds is less than the recoil reaction on the scope mass 594 pounds, the scope should slip and the screws should start shaking loose.


Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: August/22/2015 at 09:14
urbaneruralite View Drop Down
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Someone ought to write a textbook called Practical Physics for the American Male. It should teach following a model of examples as above to show how the seemingly abstract concepts apply to things young fellas might actually care about.

I bet it would sell to the home school crowd. I'd sure buy it!
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: August/22/2015 at 11:24
Steelbenz View Drop Down
Optics Master Extraordinaire
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ROLL TIDE ROLL

Joined: January/03/2006
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Bed your bases and use blue loctite.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: August/22/2015 at 11:27
Rancid Coolaid View Drop Down
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That all looks very complicated, and I am a dumb ol' Marine.

I can say this: I have fired hundreds of rifles, many thousands of rounds, and have had almost no problems whatsoever with scope bases moving or coming loose.  I have mounted hundreds of scopes, have had some rings-to-bases shoot loose, some ring halves loosen, almost no bases loosen. The exception to this was a few Tikka's with terrible stock rings. And one 300WM that was shooting 190-gr rounds at about 2900 FPS from a 7-pound gun, it had a brake, which means it was getting the double-whammy on recoil, it had a base that needed adjusting. Once.

My experience has ranged from 22LR up to 50-cal, I just don't have issues with bases shooting loose.

I am a bit concerned that you are mounting barrels but not checking torque on the bases prior to going out for test firing.  Of hundreds of rifles, "half the time" you get lousy groups due to bases: if that statement is even partially accurate, you are doing something very, very wrong.

Are you familiar with loctite (more info http://www.loctiteproducts.com/)?

Are you using quality bases?  You mention weaver, but I don't know if that is the style or the brand, this could be part of the problem.

On the physics, college physics was a long time ago for me, but I seem to recall that screws make the calculations a bit more complicated, and an adhesive makes it even more difficult.  I applaud your efforts at analyzing the problem, but just don't see the problem in the same light.

You are re-barreling actions, regularly, and having difficulty with scope bases?  I am not understanding some part of this conversation.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: August/22/2015 at 13:42
338LAPUASLAP View Drop Down
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Scope Swapper

Joined: October/17/2009
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Agree.

Many new base combos use loctite now suprisingly I have not used the supplied loctite and have put a few down the barrel without getting loose screws. Some I even use neverseeze or a similar product due to the fact I do lots of swapping... Properly torqued even with the heavy hitters on SLR or autos and no issues.

Follow the base and rings or base/ring manufacturer instructions. Use the proper amount of torque, get the wheeler or the borka and do not guess.

Dis regard the very first post because if you follow the correct procedure you will not have loose screws on your rifle like the loose screw up top, and all the mechanical enegineering and all applied physics in the world don't matter a hill o beans if you cant follow directions.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: August/22/2015 at 15:45
Son of Ed View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: August/22/2015 at 18:06
Sparky View Drop Down
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Are you using a torque wrench to tighten your bases and rings?
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