I'm in the process of planning a 14.7" build to host an AEM5 suppressor I have on order for my MK 12 Mod 0. I've worked out the math I believe the following is accurate and I'm having Ron Allen review it (great guy) to make sure I'm right since this is a pin/weld build and once it's together it's together (I know it can be removed but its a PITA).

ADCO states on a 14.5" barrel that the collar's shoulder is around 12.1" form the front of the receiver. The 14.7" brings that up to 12.3", however to account for the longer collar needed for the M4 AEM5 adapter, I'll lose another .625" back towards the receiver, landing somewhere around 11.675 then another .05" for the suppressor to overlap the collar. So max rail length is 11.625, or rather a few thousands shorter.

I'll also be using an 'F' marked FSB with carbine gas. I figure a 2.175" or so gap from FSB to the rear of the supressor.

I need some help deciding on a hand guard. DD Lite 9.5 would be awesome if I could find one used. Just missed a 12" FSB lite rail for $185 that woul d of been nice, which I know is too long but I considered trimming a little off the end hence the reason it'd have to be used. Cant bring myself to chop up a pretty new rail.

7" BCM KMR Alpha is in the mix, cause I want a light upper. Anybody have a pic of a 7" KMR Alpha with a FSB and 14.5'ish barrel?

What rails am I overlooking that you guys would recomend?

Thanks!

if you want light it cant go together in the same sentence FSB quadrail.

Spikes handgaurds?  Some good options in their catelog for the lengths you're looking for.  http://www.spikestactical.com/rails-handguards-c-146_152.html?page=1&sort=20a
Pricey, but very nice.  IIRC you can source straight from the manufacturer, instead of purchasing through Spikes, to save a couple bucks.

Check out Seekins Precision BAR and the Seekins Precision SAR. They are really similar to the Spike's handguard.