I went to remove the Talley mounts on my .416 Remington earlier tonight to try a cross slot mount instead. I found the front screw of the front mount turned without resistance as I unscrewed it. I thought it has loosened itself but after removing the rear screw and pulling off the mount, I found that the screw itself had simply sheared off. Thankfully I was able to grasp what little was exposed of the screw with some tiny vice grips. Going slowly and using some penetrating oil I was able to twist it free of the receiver and save a what for me would be an ugly tapping job.

By my reloading book this rifle has 450 full boogie loads and another 100 plinker light loads up the pipe. Of course I don't know which of those shots cracked it, and it might have been that way for some time. I will leave it to the mechanical engineers to figure out the shear forces at work but it did make me think "wow!". The scope on it was a Leupold VX III in 1.75-6x32 which isn't super light but certainly is not a heavy scope. The mount screws in question were the standard 6-48 size. My question for those of you who shoot things bigger than little .416 is has this happened to you? Did I just get unlucky with a bad screw or does this eventually happen with recoil stress. Do you re-tap to the 8-40 size screws as a matter of practice?

Yes,
That is why we routinely 8x40 those screws on a heavy kicker.
Talley 6x48 screws are not immune, even if they are best quality machined steel.

I have it done routinely to smaller bores that will carry heavy scopes, or any rifle whenever I have the opportunity.
Marlin 30-30 & 45-70, etc., rifles come from the factory with 8x40s, so do Kimber Montana .308Win "sheep rifles," etc.

If you want to forego the drill and tap, use J-B Weld epoxy between the bases and receiver.
I do that even with 8x40-screwed bases ... almost as good as an integral base then.

If you want to remove the epoxied base, take the gun out of the stock, wrap it with wet cloth rags, and apply the propane torch flame to the base just long enough to soften the epoxy, remove the screws, and slip the base off.

Millett 6x48 base screws are cast and look like powdered metal when they break. I quit using those on anything but a 22 rimfire.

Never a bad idea to go 8x40 but OTOH, I've got 6x40 on all my guns including 458 AR and 458 Lott with no problems. Maybe I'm just lucky. That may have been a flaw in screw and not a recoil problem. May have broken when installed. After all, the rifle shot fine with three screws, no?

I don't trust those dinky little screws on anything bigger than a 223 My smith charges $40 to drill and tap. Seems worth it to me.

I've had 6X48 screws fail on guns with heavy scopes. Years ago I went to 8X40 on all my big bores. I've had 8X40 shear off of .50BMG's so its not a foolproof fix. It's always agood idea to check all mounting screws on a yearly basis or before a match or hunting trip.-Rob

Thanks, I honestly I did not think it would happen so quickly.

I am not a mechanical engineer (nor do I play one one TV) but the force to shear it off on a properly torqued screw base in full contact with the receiver has to be significant. I suppose that it is be the repeated hits that lead to a small fractures and then the overall failure though. Simply measuring pure shear strength is not going to give you the real picture. I will be retapping my receiver and drilling my mounts to take the larger screws.

On a related subject the Talley rings were the screw type detachable and once I had them off I found that the front rings screws of both pairs of rings were buggered. I twisted off several T-15 Torx bits trying to get them to budge and they would not. I suppose they got banged with the little bit of scope play. I ended up cutting the rings off the scope with a Dremel. I managed not to touch the scope but it was not fun.

The whole point of the exercise was the Talley Rings even in the low size that I have sit too high for my stock and I get whacked a bit in the jaw with recoil. I wanted to try a low Burris Z ring and also a low Leupold PRW to see how they fit me. Does anyone have an opinion on the Z Ring or PRW ring choice for a medium heavy kicker?

I'm wondering if the screw was cracked or damaged on installation, and the recoil jarring just finished it off. I'm just wondering how only one screw would break. If the other 3 were still solid, there could be no lateral movement in the scope/rings/bases in relation to the rifle.

Also, I've had the same problem with stripping a few torx heads on Talleys. Sometimes they really lock up if tightened heavy. I just used a drill bit, smaller than the head, and drilled in until the head was drilled off. The scope ring then will pop apart, and you can back the rest of the screw out with your fingers, with absolutely no damage to scope or rings. Talley will send you as many replacement screws as you need, free of charge.

Well I am feeling a little foolish now for cutting off the rings when I could have solved the problem by carefully drilling the head off. I think I was really fed up with them at that point as I had managed to scratch one of the rings quite nicely when one of the bits broke off. At least I know I have pretty steady hands with a Dremel and a cutting disk now.

I don't think any of the other screws were as tight as they should have been in retrospect. I tighten base screws to 25 inch pounds of torque, and they were easy to remove. Perhaps it did move around? In any case I will be upgrading to the larger screws and use some Blue Loctite this time.