Re: Base screw lenght
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Sunnyside Scott</div><div class="ubbcode-body">I am not a gunsmith but I have read books on structural engineering.
Here is my take, most of the time the screws are going to be subjected to shearing, (wanting to be cut in half by the scope and rifle going two directions), longer screws won't help.
Next would be the forces subjected by the scope and base if it were to bo torn away from the rifle. Do you think that you could have a mishap where it would tear the threads out of four holes? I doubt it considering that you had them torqued down and didn't tear the threads out in the process.
You didn't really mention how deep they threaded down, as that it could still be an issue if you only caught a few threads. In the end it has more to do with your confidence level, if you think it needs longer screws, get em, there cheap.
SScott </div></div>
There are other engineers on this forum, like bohem and Jon A, that should straighten this out, not me, but I have waited, and nothing so far.
A 300 Ultra Mag from Remington weighs 7.3 pounds.
A 5x20x50 scope from SWAFA weighs 31.39 oz = 1.96 pounds
A 220 gr bullet @7000 gr/lb = .0314 pounds
A 300 Ultra with 220 gr bullet and 97.6 gr H870 will have peak acceleration per Quickload between .453 ms and 1.052 ms and the bullet velocity will change from 453 fps to 2356 fps.
Change in velocity of bullet = 1903 fps
change in time = .599 ms
Total weight of rifle and scope = 9.36 pounds
Change in Rifle and scope velocity = [change in bullet velocity][bullet weight /rifle and scope weight] = [1903 fps][.0314 pounds/9.36 pounds] = 6.4 fps
Scope mass = 1.96 pounds / 32.2 ft/ss = .0609 slugs
Force = Mass Acceleration = .0609 slugs 6.4 fps/.599 ms = 650 pounds
If that shear is shared by 4 screws, then 162 pounds shear/ screw.
If screws have a minor diameter of .138".
Area = pi r squared = 3.14 [.138/2]^2 = .015 sq in
Shear stress = F/Area = 162 lb/ .015 sq in = 10,800 psi
American made cap screws have a tensile strength of ~ 180,000 psi tensile strength.
Shear strength should be ~ 60% of that = 108,000 psi shear strength.
It would seem that the shear on the screws at 10.8k psi is only 10% of the 108 kpsi they are capable of.
But that is not what is really going on.
The screws are torqued to half their tensile strength.
That clamps the mount to the receiver with a force of thousand of pounds. There is a high co efficient of friction between the mount and the receiver. They never slip. If they never slip, the screws never see the shear forces, friction between mount and receiver see that shear.
