Will a suppressor fit on 15" KMR Rail with 16" barrel?

Theres no way I could fit any of my silencers under any of my key mod rails.

Yeah, most silencers won't fit under most key-mod or M-lok rails, and definitely not the KMR. Luckily the 15" KMR on a 16" barrel leaves plenty of room in front of it if you have a normally-mounted QD or direct-thread silencer. If you have a reflex (slip-over) silencer like the OSS cans or the older AAC SPR/M4 or Surefire FA556AR, it won't work because the silencer mounts back over the barrel a ways. But reflex cans are mostly out of fashion these days and most major manufacturers don't make them anymore.

Here's a 16" BCM with a 15" KMR. As you can see, there's plenty of space in front of the rail for a normal silencer like a Surefire SOCOM, SilencerCo Saker, AAC M4-2000, etc.:

http://shop.bravocompanymfg.com/BCM-RECCE-16-KMR-A-Carbine-AR15-p/bcm-carbine-750-790.htm

You are cutting it damn close. But to be frank the suppressor model would be vital. I bet they would have the answer as well. Chances are you are not the first to ask.

It's close, but it will be fine with every direct-thread silencer I know of, and it will be fine with almost every quick-attach silencer currently on the market. Very few quick-attach silencers extend backwards over the shoulder of the barrel threads, and if they do it's only by no more than 1/8". So as long as your rail is at least 1/8" short of where the crush washer is currently, you'll almost certainly be fine.

If you tell me what silencers you're thinking about getting, I might be able to tell you for sure; I've gone over this very thing many times with customers.