Reason I asked:
I was faced with this back in 2004. I have a friend in the Seattle area who's an ME. He put me in contact with a colleague of his who's sole job in life is developing fasteners for airplanes.
He was kind enough to work up a full evaluation based on an assortment of fasteners I sent him. Some were from Brownells and stuff made in house.
After all sorts of testing he wrote me a full report on this.
40lbs. No more, no less. More yields the threads. Less doesn't do the job.
If it takes more than 40, something is wrong. I'd start with the stock.
The issue lies in the loading on the thread flanks. A screw is a spring for all practical purposes. Your putting the fastener under tension so that it pulls the receiver into the stock. If the load exceeds the capacity of the threads, it will yield and the tension erodes. Not necessarily all at once either. The loading also needs to deliver a sufficient amount of friction so that the fastener stays put. Especially when subjected to vibration. Surface finish has a lot to do with this too. So does the class of thread but I don't get too overly concerned with that part. It is just a dumb guard screw after all. I'd have to dig up the paper, but if memory serves me correctly, 40lbs delivers something like 1600lbs of tensile load. This loading has to be factored based on the root diameter of the fastener. To put this in simple terms. Imagine a bolt measuring an inch in outside diameter. If we were to put some sort of ridiculous pitch on it that resulted in the minor diameter measuring at a 1/2" we couldn't generate calculations based on the outside diameter. The bolt would fail. Pitch and flank angles also plays into this but that's getting really silly.
Keep in mind also that 40lbs,50, or 60 is sort of a misguided way to do this stuff. Friction will skew the desired result. A screw made from 303 series stainless is quite gummy/sticky compared to one machined from 4140 heat treat. Stainless fasteners in stainless actions only accentuates the effect. Lube on the threads may help some. If we can grease up a barrel tennon, stands to reason you can grease the guard screws. don't forget under the head of the screw also as it has quite a bit of surface area as well. This is one reason why in auto racing things like rod bolts aren't torqued anymore. They measure the elongation of the fastener when it's placed under load. The "stretch" has been calculated to deliver the appropriate amount of tension to ensure the rod cap stays put. Torque to yield fasteners kinda fall into this realm too.
I've used 40lbs for at least 8 years now. Never once has it failed me. I spent 3 years in Iraq as a security contractor. I was also the chief armorer/firearms instructor at the US Embassy in Baghdad. I bring this up because I heard all sorts of rumors about M40's having issues with zeros.
Guess what the Marines preach? 65lbs.
I always wondered if that had something to do with it.
Take it for what its worth. Just thought I'd share that.
C.
Chad Dixon
Owner/Gunmaker
LongRifles, Inc.
