Sig says they are 6-40...and that they will soon sell a sealer plate kit that comes with that size (x 0.41").  CWPWS sells a filler plate that comes with that same size.  Other sources on the net confirm that is the size.

Except it's not, for me.  And for at least one other guy on SigTalk.

A 6-40 screw goes in perhaps half a turn and then binds.  At first, I thought CHPWS shipped the wrong screws, so I ordered 6-40 screws from another source, but the same deal.  Sig's solution is to send me yet another a set of screws, but they are on backorder...and I don't expect a different result.

I've tried other sizes and come up empty so far.  I have a handful more sizes on order form a supplier, but if they don't work, my choices are either try forcing them in (and potentially needing to retap, without much spare slide available) or send the slide to Sig for them to figure out what's up.

But I'm curious, anyone else have a similar experience...and find a solution?

Sorry I’m not much help, I just ordered my plate and am probably going to be in the same boat as you when it comes in.

Did you find a solution?

Not yet, but still working it. I'll update after I figure it out.

Trip to Home Depot to try different screw threads.

Even though they have an entire aisle of fasteners, their selection of machine screw sizes is actually pretty small.  Sure, if you want 4-40 or 6-32, you are set, but try getting 6-40 at HD or Lowes.  Neither have it, and the supposed fastener specialist at my local Lowes told me there is no such thing.  Not they didn't have it.  No such thing anywhere.

So yeah, I have been ordering online.  


Thus far, I have tried:

6-40 (two different sources)
6-32
5-40
5-44
4-48
4-40
M3
M4

And just got M3.5 in today from McMaster Carr...but won't be able to test because I got tired of dicking around and shipped the slide off to get the real answer.

That sucks, my plate and screws worked perfectly. Seems like a Sig issue.

The screw holes are 6-40. Unfortunately we ran into this issue this weekend, also. A friend bought a new SIG 320 carry with the X-series slide. The starboard side screw hole above the extractor had no issues-- a 6-40 screw threaded in just fine. We shortened the screw so it wasn't encroaching on the extractor spring channel and moved on to the port side screw. The 6-40 screw started about 3 threads and then became very snug. We worked it back and forth several times, but eventually the lower threads on the screw demonstrated deformation and the black oxide finish was stripped off. It appears that the tap that formed the screw hole was worn out and did not get replaced as soon as it should have been.

Luckily I had a couple of 6-40 taps to run down the hole. The 6-40 taper tap went in with no problem and bottomed out after barely touching the threads at all. I then followed with a 6-40 bottom tap and sure enough, I started cutting threads right away. Unfortunately, I broke the tap about 1/8 of a turn before I was going to call it quits. :(  There is a company relatively close that can burn the tap out using an EDM, so the project is on hold until that can happen. For what it is worth, the tapped hole in this particular slide is approximately 0.25" in thread length when fully threaded. This was a brand new bottom tap and I was using plenty of cutting oil and going, very, very slowly. These taps are just so small and the material/coating is so hard that breaking a tap is a very likely scenario.

Having now experienced the problem first hand, I would recommend calling SIG and ask them about fixing your slide rather than doing it the hard way, like we are doing.

Thanks much for the response.    And this situation is exactly what I suspected after having so many people tell me they should be drilled/tapped for 6-40 and striking out with that size and all the others I tested.

And I have called Sig.  And emailed them.  And while they have been responsive, they have been zero help.  They have allowed for zero possibility the tapped holes are out of spec.  Both the telephone and email customer service representatives I've spoken with have insisted that the problem could not be the slide and that I must be using the wrong screws, even though I've explained, multiple times, I have tried 6-40 screws from different sources.  I have requested an RMA multiple times and in response they have simply said that the problem is not the slide...offering to sell me their new sealer plate, which comes with 6-40 screws. At one point, one of them changed his position and said my slide wasn't "cut for the RMR."  I had to send a picture to prove that it was drilled/tapped for one...and then he went back to telling me I just didn't have the right screws.

So while you are right, Sig should be dealing with this, I am done with them.  CHPWS has been great during related exchanges I have had with them and they stepped up to say they would take a look, so I shipped it to them.

I have been a shameless Sig fan since the 90s, but this experience has been extremely disappointing.


BTW, before sending the slide to CWPWS, I did eventually decide to use a bit more force on the 6-40 screws.  With moderate force on the left RMR hole, I managed to get the screw to go in, but with the same result you had - screw threads deformed and oxide stripped off.  Fortunately, the tapped hole didn't seem worse for the experience.

I tried using the same amount of force on the right RMR hole (near the extractor) and the screw wouldn't go in, so I stopped.


ETA:  I have been talking with a guy on SigTalk who has the same issue with two recently purchased slides...and his results with Sig CS are the same.  They insist he must have the wrong screws and that there is zero chance they have a QC issue.  He was, however, eventually able to get Sig to agree to look at it, so this will be interesting, especially since Sig has said they don't have the correct screws in stock.  Hopefully they have changed the worn taps and can check that way.

Interesting, I had this problem as well. Except I thought the problem was with the CHPWS screws or mount so I returned them. I guess that wasn't the problem after all.

I don't think Sig has any idea what is going on here...and they are sticking their heads in the sand.  

The guy I am speaking with on SigTalk has two new Sig slides with RMR holes in which the 6-40 screws do not fit and a third slide in which the 6-40 screws do fit.  But Sig refuses to acknowledge there is a potential issue.  In fact, today they doubled down with me, insisting that I need their kit and that's my problem.  Their kit that has 6-40 screws.  Apparently, those are magic shape-shifting 6-40 screws...but who knows, because it's new and not available.  And you only need to scroll up to see another guy who claims his two RMR holes were not even tapped the same as each other.

I'll update this thread after CHPWS analyzes my slide...and after I get info from the other guy on the slide(s) he managed to get Sig to agree to look at.

I wonder if it would be less trouble at this point to just buy a plain slide and have it custom cut to whatever I want...

There a quite a few third-party vendors who sell P320 slides already cut for an RMR, including Agency Arms, Grey Ghost Gear, Zev, Norsso, Killer Innovations, Syndicate, True Precision, and Live Free, with LF being perhaps the least expensive.  I'd go with one of them before I paid for a slide cut/mill.

I was trying to get a deal by getting an XSeries slide with a Romeo1 Pro and selling the R1P, since they include suppressor-height sights on that slide.  They don't on the slides they sell with just the cut.  They also sell all of their slides with striker and extractor assemblies, which most of the other folks don't.

So yeah, it was a great plan.  Until it wasn't.

Broken tap update: Took the slide with the broken tap to Stillion Industries/Electro-Arc in Dexter, MI and they used their Metal Disintegrator to burn out the tap in a matter of minutes. Cody did a superb job-- the slide is pristine and the threads are undamaged all the way to the bottom of the 6-40 hole. I had unknowingly bottomed out the tap, so the blind hole was completely tapped to the bottom with close to 0.250" thread engagement. After taking the slide back to the house, we cleaned it well and removed all remaining tap bits from the hole using a pick and a magnet before applying acetone to remove any remaining oil.

This time the 6-40 screw went all the way to the bottom of the hole with no thread damage. My friend decided he did not need/want a plate under the Holosun so we cut the port side screw to 0.450" in length and the starboard (ejection port side) to 0.391" in length, wiped the screw down with acetone, applied Loctite 248 and torqued them to 15 in-lbs (the SIG torque spec vs. the 12 in-lbs that Holosun recommends.) We will give the Loctite a few days to cure before taking it to the range but we are confident that we have solved the threaded hole issue and are looking forward to testing the 320 with the red dot installed.

Side notes:
I chose to use the C&H thin plate under my Holosun when I mounted it to my 320, but there is 0.025" clearance between the back of the plate and the slide and there is some clearance in the front as well, so I really question if the plate is providing any benefit securing the Holosun to the 320 slide. That is why my friend decided to direct mount his Holosun 507cV2 directly to the 320 slide without any adaptor plate.

Stillion Industries was a little difficult getting hold of but we succeeded by phone and were very happy with the results. Their phone number is 734-475-6016. Ask for Keith (the owner) or Cody (the sales rep/tech that sells and demonstrates the Electro-Arc metal disintegration machines.) They quoted us $60/hour, minimum of 1 hour to burn out the tap. Cody actually showed us around and let us watch as he burned out the tap. Like most jobs, the set up was the most time consuming part of the job as he wanted to make sure he was dead center on the tap to make sure the slide was not disturbed. It ended up taking maybe 30 minutes from start to finish. I was amazed to learn that they can burn a tap as small as 0-80 (that is tiny!) and as large as 2 inches in diameter so a 6-40 tap was pretty easy-- especially on a slide that had nice flat sides to index on. (And yes, Cody was a pro-- he used brass shims to protect the slide finish and he made sure to tell us to clean the slide well even though he used a water soluble oil flushing media and dried it off well using compressed air.

After all this, I would currently recommend sending a troublesome slide back to SIG to be fixed rather than doing it yourself, if possible. I will be very curious to hear what experiences the others above have when they get their slides back.

Glad to hear you got the tap out.  

Send to Sig?  Well, I am in touch with a guy who did just that, after he also could not get 6-40 screws from CHPWS or Monster Bolt to fit.  Sig apparently examined the slide, told him the screws fit, and that they were sending the slide back to him, with screws.  And they sent him M4 screws, for the R1P/DPP mount holes.  

I sent my slide to CHPWS instead and they found 6-40 screws that did fit - hex key head, unlike the Torx head they sent originally.  I have no idea what's up, but I cannot deny that they screw in fine.  No force, no wiggle.  The set they originally sent me did not fit, nor did the set I got from Monster Bolt, so your guess is as good as mine.  I'd rather have Torx heads, but at this point I am just happy to have a set that fits.  There are not a lot of options for 6-40 screws.


Do I owe Sig an apology? I honestly don't know, but it would have been really helpful if they could have sold me a set of screws to address this issue, which they apparently still can't.

But a shout out for CHPWS for their help and finally getting it solved.  They have been helpful the entire way, and did this analysis and shipped the slide back for just the cost of a pair of screws, which they also filed to length, as to not interfere with the extractor.  And their thinner filler plate is excellent.  It snaps in snugly, makes the RMR fit snugly, and has recoil/boss lugs as well.  It's also thin enough to allow me to co-witness the dot with the Sig suppressor-height XRay 3 night sights that came with the XSeries slide.


The end result:

Excellent that C&H was able to help you out and your pistol is up and running too. For what it is worth, the screws that came with my thin C&H plate were also hex drive and not Torx (and also marked YFS on the top). For what it is worth, you can get spare 6-40 screws from FMW Fasteners (they are marked YFS on the heads, as well) but they are also hex drive. Interesting that your plate is a snug fit to the slide-- wish mine was snug to my VTAC, but we will see what happens long term. As you mentioned, the Holosun seems to be made for the 320 as it sits down nice low in the sight plane-- noticeably lower than on the Glocks. That is a real bummer to hear that SIG was less than helpful for the other guy. I guess my new recommendation is to buy some 6-40 screws and take them with you to the gun shop when you are looking to buy a 320. Take the optics plate off and make sure the 6-40 screws thread all the way down in the slide before purchasing.

BTW, during this process I tried both the CHPWS adapter plate and filler plate for a P320 with R1P/DPP cut and measured their thicknesses.  

The adapter plate is about three times as thick as the filler plate, and resulted in the RMR blocking a good part of the Sig suppressor-height irons:





If I stuck with it, I was going to end up needing to head over to Dawson Precision to get taller sights.  On the positive side, you don't need the slide to be drilled for an RMR.  Or spend weeks trying to hunt down screws for those holes.


The filler plate, which does require the extra set of RMR holes: