if you did not make the connection SoCalDep is the expert in the link I sent you. I trust his opinion on this topic and have been following his advice in my own endeavor.
Also... Hey Trey.
Hey Ethan... Hopefully you're doing well and your program is continuing to move forward.
As for the screws not seating... That's an issue with 9mm and the likelihood that a warm 10mm round will exceed 9mm is real. That said, I don't remember a 507C mount failing - They are all 508Ts that came loose, so you'll be a good data point.
To answer the question about the stainless screws vs alloy, we need to discuss how the screws work and what forces act upon them. There are three primary forces that are occurring when you're using screws on an optic, and that means three points of potential failure. Tensile strength is the ability to hold two parts together that are trying to pull apart. Torque is the rotational force being placed on a fastener (screw) as one tightens it down, and shear force is the force placed perpendicular to the direction in which the screw is being tightened. There's little compressive force because we're generally dealing with metal that doesn't compress. It's there but it's less relevant. In fact, it's probably relevant in getting the screw past the 507C optic body to seat but that's another conversation and a whole lot more experimentation.
So the McMaster alloy steel 6-32x1/2" screws have a listed tensile strength of 140,000 psi.
The McMaster stainless steel (18-8) screws have a listed tensile strength of 60,000 psi.
Depending on what engineering formula/theory you use, which I believe if I remember correctly depends on the ductility of the steel in the screw, you're looking at around 57 or so % of the tensile strength to get the shear strength. This means that in a very oversimplified way the stainless steel screws have 42% of the tensile strength and shear strength of the alloy steel.
That's a thing, and I think the increased strength of the alloy steel screws will provide a potential early warning to a loose screw prior to catastrophic failure and shearing.
It doesn't prevent the screw from coming loose, and a loose screw is the problem. Again, my experience is with 9mm, but I believe that a screw does not shear on an optic from tensile forces. If mounted properly with a good mounting system as long as it's tight the shear forces are primarily on the optic against the plate and not the screws. Once an optic comes loose, the shear forces are enormous. It doesn't take long for a screw to be fatally injured.
So this brings up what that "fatal injury" means. A screw is like a very rigid spring. It has a certain amount of deformation that is "elastic" meaning that it can return to it's original dimensions. Once the screw is stressed beyond it's elastic range and reaches plastic deformation it cannot return to it's original dimensions and it begins to be damaged.
As we have seen, there is fairly minimal tensile forces against the screws, so they should hold to that force. The torsional forces, so long as they don't exceed elastic deformation dissipate rapidly, so tightening the screw doesn't hurt the screw unless it's beyond the screws capability to begin with. Shear forces are high, but as long as the optic isn't moving with the slide's reciprocation that can be controlled. Since we don't have compressive force (of any real measure) we need something to keep the screw in place and tight. That's where threadlocker comes in. The purpose of a torque spec is primarily to lock the minimally compressive materials (the optic body and the slide with maybe the sandwich of a plate) together without exceeding the elastic deformation range in strength of the screw regarding the torsional forces. If that were to happen, the screw would loose strength rapidly as the - in my words - critical line - was crossed and exceeded. At that point the mount is doomed from the start. That's rare.
Lots of words... what does it all mean?
Tensile (and the related shear) strength of the screw is less important than proper mounting. If the mounting stays tight then the system should work.
I see several issues in the previous posts in this thread... things that we have done and encountered - the mistakes of our past.
First... The screws from C&H were not too long. They were probably not the same thread pitch. The M&P has through-bored threaded holes. You can use a pretty long screw, but not if it's the wrong pitch (they use something like 6-40 on some of their stuff...). I've experienced C&H sending the wrong screws before on more than one occasion. This couldn't have been great for the threads in your slide. Add to that you ran a tap, which may have corrected the initial damage, but if the tap was on the large side of spec may allow more "looseness" and reduce how tightly the screw is able to secure the optic.
So, ideally you now have good screws installed correctly on your slide/optic, but if they don't have good 360 degree contact with the optic body, and they are proud of the pocket because the aluminum body has prevented the screw from tightening down, that small surface area of the screw has the potential to move the metal on the aluminum optic body when the slide reciprocates. It's too small of a contact area and in a 10mm I sure wouldn't trust it. I've seen that several times on multiple platforms in the past. As soon as the aluminum (being softer than the alloy steel) deforms enough to allow the optic to move... it is done. The optic will come loose. The shear force will begin to compromise the integrity of the screw, and if not caught early it will shear. Even alloy steel.
How do I know this? I and several other members of our unit fired 1,985 rounds in about 20 minutes through an M&P with a Trijicon SRO mounted with McMaster alloy steel screws. It was so hot that we poured water from a hose down the ejection port/barrel and on the slide to cool it when we couldn't shoot it any more. The heat (I think) ultimately loosened the threadlocker, which allowed a screw to start backing out. This lead to peening of the plastic plate (now things are moving with much more force), and when doing a final accuracy test we noticed a significant zero shift. I saw the indicator marks had moved (more on the right side) and went to the armory to check. I tried to remove the right screw and it began turning (probably the top against the cracked/sheared section) and broke free. The other screw came out intact.
The recoil forces were no more than the several optics I've mounted with stainless steel screws that have higher round counts. The problem was that the screw came loose.
achmayr Master Gunsmith Screw Kit #03061
