Can someone share a photo of how the rails mount? I have a vanilla M249s which I'm inclined to add a mk48 rail (tripod mount necessitates this vs the mk46) but there's some drilling involved and I'd like to minimize the jankiness. Are the holes in the receiver themselves threaded? Nuts on the inside?

ETA 1 - This appears to be the most accurate photo I can find at the moment... Seeing the three holes that handguards mount to and two slots I suspect for the lightening goals:



Also seeing receivers that are only slots (allowing handguards to slide presumably?) as well as receivers that only have the three holes.

ETA 2 - Stumbled on a blurb adding another SAW to the mix. The M249 SPW was the precursor to the mk46 and uses the slot only configuration for the top heatshield. No provision to mount the top or bottom-facing KAC rail from the midline. mk46 adds the holes for the larger KAC top or bottom rail to fasten. Starting to get a little clarity here!

Sounds like you are asking specifically about mounting a KAC rail, but since no one has answered I’ll offer this.  The “M249 Accessory Rail Kit” NSN: 1005-01-490-9698 looks like the picture below.  I have mounted and disassembled this rail before.  It is a 2 piece rail that clamps to the sides of the receiver, and uses circular protrusions machined on the inside of the rails to interface with the circular holes on the receiver. No drilling is required and it is rock solid. Mounting screws run horizontally through the bottom 6 o’clock rail section and don’t touch the receiver at all. You will most likely have to find it on the “surplus” or second hand market, but they are out there.  They are issued with KAC rail panels and a KAC vertical grip as part of the package.



Edit to answer one of your questions I missed. The holes on the receiver are not threaded, nor do they have nuts behind them.

OP - are you referring to the upper or lower handguard?  The poster above responded about the lower handguard.  The upper mounts as you mentioned on a proper MK46 Mod1 receiver.

FWIW, I have an upper handguard listed in the EE:  https://www.ar15.com/forums/equipment-exchange/M249-handguard-and-railed-upper-handguard-PICS-/118-2238670/

Hey guys! Thanks for weighing in. I ended up going with a mk48 lower rail to get around the M249 mounting lug. Same holes so it's either mk46 RAS up top or mk48 RAS on bottom. I prepped the holes but skipped the slots since I have no use for them at the moment. Anyone looking to do the same - I ended up confirming the following alignment:

mk48 RAS, for lack of an official name, flush with top of receiver...



...and flush with the muzzle-end of the heatshield.



Overall way happier with this setup than the B&T rail I had installed before.

Nice!