I’ll add on to this for anyone else looking to go the route of an RMR on a 43. I’ve done a bunch of sifting through the interwebs and some testing of my own and here is what I’ve come up with.
There are really only 2 ways I have found to mount a Red dot on a 43/43X/48, just like a full size Glock.
1st is the Dueck Defense / Competition style of Dovetail mount.
Pro's: Not a permanent attachment method, cheaper, can be mounted at home with proper tools.
Con's: Dovetail is a potential failure point, RDS sits very high, no back up irons unless it's the Dueck mount.
2nd is the milled slide. Within this there are three options I have found.
A) Shield RMS/RMSc/RMSw and whatever else shares that foot print.
B) Trijicon RMR/Holosun 507c via an adapter plate to widen the slide.
C) Aimpoint ACRO direct milled.
I'll address each of these with my own thoughts and experiences.
I had a 43 milled for a Shield RMS by Maple Leaf Firearms that I thoroughly enjoyed using. The optic sits so low that standard 43 sights co-witness enough to get a back up sight picture. I loved that with the standard RMS I could change the battery without removing the optic. I disliked the lack of tactile clicks when adjusting the impacts. However, through a stupid mistake of my own, I managed to drop the 43 from my lap onto the concrete of the garage floor. Of course it landed glass side down and a lovely crack went straight through the polymer window. Since I sold this slide, Maple Leaf has released an adapter plate that fills in the Shield RMS cut out and allows you to mount an RMR above it. I'm not sure how high your BUIS need to be, but it seems like it could be a solid option for using an RMR when you already use a Shield RMS, and it has recoil bosses which I'll talk about next.
I am currently running a slide from Suarez international with their NSA plate for the Glock 43. It allows me to run a Type 2 RMR on my 43 which is my go to optic on my other full size Glocks. All of the stuff with Gabe aside, my major complaint is a total lack of Recoil Bosses on the plate. The pocket for the plate is longer than the RMR so I switched the Rear sight around in order to give a third point of contact against the RMR back. I have witnessing marks all around the optic, but I just have a bad feeling the zero will shift. I'm intending to go to the range and give it some bump testing to see, but I will say it has not failed after the 500 rounds I've put through it. Since I purchased it, I have found out that MAC Defense Industries
l&m precision gunwork and Primary Machine both also offer an adapter plate method similar to the Suarez, but their plates both have recoil bosses. MAC defense does an Irons forward option that I would have preferred.
Finally is the Aimpoint ACRO. Personally I would have loved to do this route, but I can't get behind the short battery life. However, having an optic that I could beat the crap out of with an enclosed emitter and replace the battery without removing it would be amazing. I have seen ATEI and Victory First both market slides for the 43/43x/48 with the ACRO. ATEI even has a SHIM sight that let's you do a smaller lighter rear sight.