I recently purchased a P365XL.  I have never had a pistol with a RDS so I decided to get a Romeo Zero and try it out.  If I liked I would later upgrade to a better RDS.

I took the slide off and underneath are two 5/64th screws holding on the iron sight and sight plate.

I could not break those screws lose.  Sig put Locktite on them and they were tight.  Really tight.  After awhile after trying to force them too much I wound up buggering up the screw heads.

I don't know if it was because the screw heads were made of soft metal or my hex wrench was not very good or I was just an idiot.

Fortunately the service rep at Sig had mercy on me.  They will take it and fix it for me.  They sent me a shipping label.  The rep said I wasn't the first one.

I should have bought one with the sight already installed.  

If you are planning on installing one yourself be careful of those screws.

Buy quality tools.

Hint: Not available at HFT.

Get some Wera, Wiha, or PB Swiss for any kind of fastener. I prefer Wera myself, but own some stuff from all three.

One way to break loose a thread locked screw is for small screws take a soldering iron and hold it to the head of screw for a few minutes or more then back the hot screw out. First make sure the Allen wrench or Torx bit fits tight, then heat it.

I installed my optic at the range, using just a quality set of Allen wrenches. I have Bonhous sets. Did not give me any problems taking off the rear sight.

One thing I did do when first installing the optic was zero it, then take it on and off a couple of times to see how it locates on the Sig slide. It stayed very close to zero, after being removed a bunch of times.

There is no thread locker on the XL mounting plate screws.  If I have time in the next day or two, I'll try to write up a guide to mounting optics on 365 XLs and 320s.  

Yea those things suck. I got one out and very carefully drilled out the stuck/stripped one. Glad to hear Sig took care of you

That was most likely the problem.  A 5/64th hex wrench is not something I have ever used very often and probably the one I used was not quality.  It was probably not a tight enough fit and when I put pressure on turning the very tight screw heads wound up reaming out the heads.

I always try to buy quality tools but evidently missed it on this one.

Not necessarily. I have three sets of USA hex wrenches, that don't have any relief, and I was able to get mine out with a few minutes of soldering iron heat. But, I did one for a buddy and his screws were not coming out. I tried all three sets of wrenches, with 5 minutes of soldering iron heat, and they weren't budging. I then got a T8 in the screw head, with a very solid bite and heat, and I finally stripped them. Called SIG and CS told me that "you weren't going to get them out" and that "they were bad fasteners". Like most of us, I've been working with fasteners since I was old enough to turn a wrench, and these fasteners were either too soft of a composition needed for the torque required to remove them, or they had a heat resistant thread locker on them. I did read somewhere that guys were having luck putting the slides in the freezer, but I didn't read that until after my failure with my buddy's XL.

Update:

I sent Sig an email to their customer service and they said they would take care of the problem.  They sent me a Fed Ex mailing label.  I sent it to them last Monday.  I received it back today (Monday).  Very fast turnaround in one week.  No charge.

They removed the boogered up screws, mounted the Romeo Zero and sent me new screws.

This was the first time I used Sig customer service and I was impressed.  Especially since I was the one that messed it up.

Since I got a pack of torx head screws for my RMR I won't use anything else. Soooo much better than allen heads.

I just drilled the head of the screw off. Took about 30sec and didn’t come close to hitting the threads in the slide. New screws from Sig are on their way, free of charge. Not that I intend to put the sights back on though…

I used the tools that came with the sight and mounted mine up no issues last week. Glad to hear Sig took care of you. The 365xl is also my first gun with an RDS and so far so good ??

I almost had the same problem. When I felt the Allen head slipping I stopped.

Swore they were Allen head screws. But I used a torx bit and they came right out.

Google says some XLs have Allen some have torx.

How did you get the screws out after you drilled off the heads?

One the heads are gone there’s nothing holding the sight down. It just lifts right off. Then you can grab the screw shafts with some channel locks and turn them out.