Mismatched Standards... Finding correct fasteners for mounting a 507c to S&W M2.0

[NC_BRP]

Good afternoon! New poster here. Hoping you all might be able to provide some feedback as there seems to be quite a bit of misinformation or misunderstood information on this topic...

I purchased a Holosun 507c to use with my M&P 2.0 Compact. S&W suggests that you use the fasteners supplied by Holosun / Trijicon but as I'm sure many of you are aware, these appear to be too short and will not allow for sufficient thread engagement.

I started to go down the rabbit hole of finding some fasteners and it appears that Holosun provides a custom fastener where the hardware itself has a metric/ISO designed screw head with a 90 degree countersink angle... but then combines that with inch standard 6-32 threads. This makes fasteners somewhat impossible to find because inch fasteners traditionally use an 82 degree head angle and means you'll never get complete contact of the fastener's head against the optic. I wrote Holosun and they suggested a few vendors but none of these vendors actually sell this kind of hardware. It's basically a proprietary piece. Yikes. Not a problem if you're mounting to a Glock tapped with metric threads and can find the correct hardware. (I'd imagine Glock folks doing this might have the opposite problem - metric threads but needing an inch-style 82 degree countersink for mounting an RMR.)

Would appreciate any feedback you all can give. I'm just trying to mount this thing but it is sadly turning into a mess trying to source the perfect parts. Given the large amount of slides and optics and footprints and metric vs standard etc, there's no doubt others have run into this problem before. Are folks just running 6-32 x 1/2" fasteners with the 82 degree countersunk head? This won't match the 90 degree countersink cut on the surface of the optic... but have there been any real problems because of this?

[whomever]

Sorry I don't have a good answer, but will note that if you know someone with a lathe, you can make an 82 degree screw head into a 90 degree one without reducing the diameter (while going the other way reduces the diameter).

caveat: I don't know what the head diameter of your hybrid screws is. It could be that a 6-32 head is already too small, or not :-(

Anyway, it's an easy thing to do with a lathe. If you were local I'd invite you over.

('Hey honey, remember that red dot? It turns out I need to buy a lathe!')



ETA: When you say 'it appears that Holosun provides a custom fastener where the hardware itself has a metric/ISO designed screw head with a 90 degree countersink angle', how sure are you? As in, is someone assuming that, or is Holosun saying that, or did someone put one on an optical comparator and verify? Could you maybe take a photo of one and zoom way in and use a protractor to check?

[NC_BRP]

Thankfully FCD put up a nice blog post on the topic which can be found at the link below...

https://www.forwardcontrolsdesign.co...un-screws.html

I've checked the Holosun fasteners against the traditional fasteners (6-32 x 1/2") I purchased locally and the difference of the angle on the head is something that can be seen by eye for sure. I'm guessing Trijicon is using an 82 degree countersunk hole on their optic and thus there's no issue sourcing good fasteners with the correct specifications for a true RMR. Likewise if one was using the Holosun optic with metric fasteners on a slide tapped appropriately, it would be easy to find the appropriate fastener. Unfortunately it is this mix between the two which seems to be the issue.

I have a sneaking suspicion that most are ignoring (or not aware of) the countersink angle difference and just matching whatever thread pattern is on their optic and letting it rip. I can envision a scenario where the screw head is not correctly seated into the countersunk hole and thus having limited contact prematurely wears due to the more malleable nature of most aluminum, then the fastener fails to retain the optic and eventually you get some shearing issues with an optic that hits somebody in the face.

Am I overthinking this?

[SoCalDep]

I purchased a Holosun 507c to use with my M&P 2.0 Compact. S&W suggests that you use the fasteners supplied by Holosun / Trijicon but as I'm sure many of you are aware, these appear to be too short and will not allow for sufficient thread engagement.

Holosun includes two to three sets of screws with their optics depending on model. I’ve installed several Holosun 407/507/508 models on M&P 2.0s with the included screws from Holosun. I did this most specifically when we were diagnosing issues that arose particularly with the 508T and the alloy steel screws we were using along with RMRs. Turns out that the Holosun optic body rides closer to the countersinks for the screws compared to the RMR and the wide diameter of the heads of the alloy steel screws were preventing the screws from seating in the pocket.

In any case, if using the Holosun screws on an M&P, you should be using the longer 6-32 set included with the optic.

I started to go down the rabbit hole of finding some fasteners and it appears that Holosun provides a custom fastener where the hardware itself has a metric/ISO designed screw head with a 90 degree countersink angle... but then combines that with inch standard 6-32 threads. This makes fasteners somewhat impossible to find because inch fasteners traditionally use an 82 degree head angle and means you'll never get complete contact of the fastener's head against the optic. I wrote Holosun and they suggested a few vendors but none of these vendors actually sell this kind of hardware. It's basically a proprietary piece. Yikes. Not a problem if you're mounting to a Glock tapped with metric threads and can find the correct hardware. (I'd imagine Glock folks doing this might have the opposite problem - metric threads but needing an inch-style 82 degree countersink for mounting an RMR.)

If that’s the case it would be dumb because it’s an “RMR footprint”, so 6-32 screws would be the standard. That said, I’m not an engineer or machinist, so I can’t measure that. What I can say is that it doesn’t matter. I’ve mounted several hundred optics over the past three years and that’s only a portion of what my department has done. Since I created our procedures, I can say that my numbers are only a percentage of what the total numbers are for mounting Holosuns over the years.

First, you’re wrong about Glocks. They use 6-32 threads in the MOS #2 plate because that is the standard RMR “footprint”. They use metric M3 screws for plate-to-slide, but that has nothing to do with the optic.

Next, I think you were either given bad information, or that information just really doesn’t matter. I’ve mounted Holosun 407/507/508 series optics on the following between October 28, 2021 and October 14, 2022:

17 S&W M&P 2.0 pistols
3 Glocks
1 HK VP9
1 Staccato

I’ve mounted at least one more on an M&P and another on a Staccato since then and probably several more but I haven’t updated my file yet. It’s been a busy six months. I used McMaster-Carr stainless steel or (for Glocks) zinc-plated steel 6-32 screws for all of them.

Now this is just a portion because I’ve mounted optics on several pistols since and a lot of pistols before, and that doesn’t count the equal and maybe greater number mounted by our armory using my procedure. Other than the wide diameter head issue on the 508Ts, we’ve had no issues. In fact, I have a 407CO mounted on an M&P 2.0 that has several thousands of rounds through it, a 407C on my Staccato that I’m shooting while teaching a class this week and that has at least a few thousand rounds through it, and another 407C on an M&P that’s around 800 rounds in. That also doesn’t count my Agency Glock 34 that has 4-40 screws attaching it’s 507C which has a couple thousand rounds on it. We tested a Holosun 407A with over 10,000 rounds and it stayed put just fine with the CHPWS 4-40 screws.

Would appreciate any feedback you all can give. I'm just trying to mount this thing but it is sadly turning into a mess trying to source the perfect parts. Given the large amount of slides and optics and footprints and metric vs standard etc, there's no doubt others have run into this problem before. Are folks just running 6-32 x 1/2" fasteners with the 82 degree countersunk head? This won't match the 90 degree countersink cut on the surface of the optic... but have there been any real problems because of this?

Don’t let the bad info get in the way of jumping into the dot life. If the longer screws aren’t in your package (if you mount them correctly they’ll work fine) then order 6-32x1/2” stainless steel (18-8) screws from McMaster-Carr (McMaster.com) and be confident. That Staccato I talked about is my sometimes carry gun when I’m not shooting the crap out of it. I worry way more about me than the gun/optic. With the only variable being my vision with the particular reticles, I’d carry either of those M&Ps or that G34 as well.

The key is to mount correctly, so here’s my way:

1. Make sure the optic works.
2. Test fit everything... screws/plates/optics - without torquing down. Make sure the plate and optic fit flush with each other an the slide.
3. Degrease all interacting surfaces. This includes the optic pocket, threaded holes, plates and their threaded holes if applicable (not with the M&P), and the screws. I use alcohol wipes but gunscrubber, alcohol, and other degreasers should also work.
4. Once completely dry, apply threadlocker. Use good stuff (NOT VC-3!!!!!). I prefer Loctite 248 but Loctite 243 is good also. With the 248 and a 1/2” screw I’ll apply to about 1/3 the length of the screw and spread with my finger so it’s even with the widest part of the thread.
5. Torque to the minimum manufacturer’s specification of the interacting components. In the case of the Holosun, it would be 15 in/lbs. If you can use a torque wrench/driver that would be best.
6. Apply indicator markings to serve as an early warning against shearing. If the optic did come loose, remove it and re-mount it with new screws according to the protocol.
7. Wait 24 hours or more for the Loctite to fully cure before firing.

[Tannhauser]

I mounted a 407 to a S&W M&P 10mm using the Holosun fasteners and the method described in this thread (which I previously read about on PF).

No issues so far.

I checked fasteners by fit checking them through the Holosun and the plate and I visually identified enough protrusion that I satisfied myself I had multiple threads engaging. I forget exactly how much engagement, but so far so good.

[3-7-77]

We issue the M&P 2.0 OR currently and I'm using McMaster-Carr 6-32x1/2" Torx Plus, Flat Head Cap Screws (https://www.mcmaster.com/products/to...ead-size~6-32/) screws on the dots that I install as an armorer.

I initially started at 20 in/lbs but had to bump that up to 27 in/lbs as some 508Ts were coming loose. I think, and this is only my theory, that 20 in/lbs is fine with an aluminum bodied optic as the screw bites in a little better. The only ones we've had come loose at the torque value were the titanium bodied 508Ts. Again, just a theory.

We're using the blue loctite in the glue stick applicator.

I'm not sure where Holosun sources their screws, but we've just been tossing them in the trash and using the McMaster-Carr's.

[SoCalDep]

We issue the M&P 2.0 OR currently and I'm using McMaster-Carr 6-32x1/2" Torx Plus, Flat Head Cap Screws (https://www.mcmaster.com/products/to...ead-size~6-32/) screws on the dots that I install as an armorer.

I initially started at 20 in/lbs but had to bump that up to 27 in/lbs as some 508Ts were coming loose. I think, and this is only my theory, that 20 in/lbs is fine with an aluminum bodied optic as the screw bites in a little better. The only ones we've had come loose at the torque value were the titanium bodied 508Ts. Again, just a theory.

We're using the blue loctite in the glue stick applicator.

I'm not sure where Holosun sources their screws, but we've just been tossing them in the trash and using the McMaster-Carr's.

The torque you are using is very excessive. Those McMaster screws are great for RMRs and SROs but the diameter of the head is too large for Holosun 407/507/508 series optics. When mounting, the edge of the head contacts the side of the optic body prior to the screw seating fully in the optic pocket.

With the aluminum frame models, the harder steel screw is able to slightly deform the metal in the optic body, allowing it to seat (maybe), but the 508T, being made of titanium does not deform and causes the “misperception” that the screw is seated when in fact it is being held with minimal contact between the edge of the screw head and the optic body. Upon firing the pistol, the screw and/or optic body is deformed during the significant reciprocating forces of recoil and once that minimal interaction is compromised the optic almost instantly becomes very loose.

If one looks at engineering specs for torque on various fasteners (screws) based on size and material, they will see that the specs listed by optic, pistol, and accessory manufacturers already commonly exceed generally recommended specs. Adding another 50%-100% is not something I’d advise and it’s tantamount to treating a sunburn with Windex. It’s unlikely to solve the problem and there’s a good chance it will make it worse…. The scary thing in this case is that the worse may just be a bit further down the road.

Because we’re attaching two relatively non-compressive materials, the torque provides minimal assistance with preventing the optic from coming loose, assuming that they are seated and “tight” to begin with. Keeping the screws from coming loose is a combination of threadlocker and preventing movement of the optic relative to the screws.

When excessive torque is applied there are a number of potential outcomes, but in the case of optic mounting none are good. Best case is that the torque does not exceed the yield strength of the material, remaining in the range of elastic deformation. In this case, the torsional forces applied during mounting dissipate rapidly, but since the materials being joined don’t compress you get minimal benefit.

If the yield strength is exceeded then the screw’s strength is compromised and the extra money you spent on those alloy steel screws is wasted.

[3-7-77]

The torque you are using is very excessive. Those McMaster screws are great for RMRs and SROs but the diameter of the head is too large for Holosun 407/507/508 series optics. When mounting, the edge of the head contacts the side of the optic body prior to the screw seating fully in the optic pocket.

With the aluminum frame models, the harder steel screw is able to slightly deform the metal in the optic body, allowing it to seat (maybe), but the 508T, being made of titanium does not deform and causes the “misperception” that the screw is seated when in fact it is being held with minimal contact between the edge of the screw head and the optic body. Upon firing the pistol, the screw and/or optic body is deformed during the significant reciprocating forces of recoil and once that minimal interaction is compromised the optic almost instantly becomes very loose.

If one looks at engineering specs for torque on various fasteners (screws) based on size and material, they will see that the specs listed by optic, pistol, and accessory manufacturers already commonly exceed generally recommended specs. Adding another 50%-100% is not something I’d advise and it’s tantamount to treating a sunburn with Windex. It’s unlikely to solve the problem and there’s a good chance it will make it worse…. The scary thing in this case is that the worse may just be a bit further down the road.

Because we’re attaching two relatively non-compressive materials, the torque provides minimal assistance with preventing the optic from coming loose, assuming that they are seated and “tight” to begin with. Keeping the screws from coming loose is a combination of threadlocker and preventing movement of the optic relative to the screws.

When excessive torque is applied there are a number of potential outcomes, but in the case of optic mounting none are good. Best case is that the torque does not exceed the yield strength of the material, remaining in the range of elastic deformation. In this case, the torsional forces applied during mounting dissipate rapidly, but since the materials being joined don’t compress you get minimal benefit.

If the yield strength is exceeded then the screw’s strength is compromised and the extra money you spent on those alloy steel screws is wasted.

Dang, I didn’t realize I was going full retard. I appreciate you taking the time to explain that in detail. I guess I need to get a hold of Holosun and order screws so I can remount some optics

[SoCalDep]

Dang, I didn’t realize I was going full retard. I appreciate you taking the time to explain that in detail. I guess I need to get a hold of Holosun and order screws so I can remount some optics

You were not “full retard”… it’s hard to wade through the different specs and then add aftermarket parts and screws to fit different footprints, threads, etc and it’s easy to find something that works until it doesn’t. We found the thing with the head diameter when 508Ts kept coming loose - we made the same mistakes.

PS - we now use the McMaster 18-8 stainless steel 6-32 x 1/2” screws for Holosuns (smaller head diameter) and they have been working great for us with degreased screws/threads/pocket and Loctite 248.

[3-7-77]

Thank-you! I’ll order up a bunch tonight. Our annual inspections are coming up so I’ll remount all the optics that come my way. My personal optic journey has been a trial and error experience.