FN P90/PS90 RMR mount

Last crawled date: 5 years, 1 month ago

Created for a friend.
This is an RMR (Trijicon pattern) mount for the FN P/PS90 and possibly Airsoft variants. The factory rail is too "tall" and this lowers the mount to just above the frame side rails where sling mounts or rails can be attached.
v1 (not published) was a for-fit version (picured in yellow PLA), that was not sized for screws properly but was a test check for hole placement and the mount hole/post spacing finalization.
v2 (not published) was a version designed for having brass threaded inserts installed.
v3 (published) made minor changes after the threaded inserts were installed in v2. This uses two M2.5x12mm countersunk screws to mount the RMR and 4 M4 screws to attach the mount to the frame.
v4 (published) replaced the hollow posts for the screws (sized in v3 to be M2.5 screws) with inserts from the top for M3 screws. This change from v3 to v4 was because the hollow screw attach points when printed with a .4mm nozzle effectively becomes about 1 layer wide so of questionable strength. The v4 is a slightly different shape at the rear so that I can see if a version is v3 or v4 when mounted and that tells me which screws to use... Also as the threaded inserts are put in from the top, rather than the bottom like in v4, the old v3 channels have been reduced to the straight through holes for other lengths of screws (or displaced plastic when you press in the threaded inserts.
For the brass threaded inserts I used two sizes from a set on Amazon (set: Initeq Threaded Inserts for Plastics Metric Assortment A, SKU IN064548-A, by/from Albers LLC)
For the mount attach points (brass threaded inserts) I used:
2x M2.5-.45mm, "long" (5.56mm long) (v3)
2x M3- "short" (v4)
4x M4-0.7 "short" (4.7mm long) for v3 or v4
To attach the mount to the frame:
2x M4x10 button head, hex screws, black oxide (Vigrue assortment from Amazon)
2x M4x14 button head, hex screws, black oxide (same Vigrue assortment) for the side to also retain a picatinny/M1913 rail piece. If you don't have a rail, you can use the shorter M4x10s on this side for a total of four (4).
To attach the RMR to the mount:
v3: 2x M2.5x10mm countersunk flat hex head. (M2.5 10mm Grade 12.9 Alloy Steel Flat Hex Socket Head Cap Black Oxidation (from Amazon, a bag of 50).
v4: 2x M3x10mm countersunk flat hex head.
I used my friend's factory RMR M1913 mount (pictured) to draft out the dimensions for the mount. This also fits the Holosun 507C or others that fit the RMR pattern mount/cut.
I don't think that the surface is going to be flat enough for the RMR to not use the sealing plate so I told my friend that.