I'm interested in purchasing either a POF, LWRC or Bushmaster Gas Piston upper receiver.  Does anyone know if there will be enough clearance for the piston mechanism if I installed a rail system on it, say, a YHM or KAC?

Thank you.

Bud

The Bushmaster is the POF system lincensed to them by POF to use and build rifles off of.
One member here has modified a Samson railed forearm to fit his POF system, but that's about all I've seen anyone do with other forearm beside using the Predator railed forearm that POF sells and designed to fit around and above the gas cylinder.
It's your best option I'd say since you'd have to dremel out a tighter fitting forearm.
I'm not sure about the LWRC and others besides their railed forearm, but you can buy models with them from either and since you'd like a railed forearm that's probably your best choice.
If you buy a Bushmaster you can always put a Predator rail on afterwards with the POF barrel nut for it.

Youd have to cut most rail systems other than the ARMS SIR type fopr clearance fop the piston.  You will be happy with the ARM-R rail on the LWRC that comes with your upper and it allows you to pull the top rail to access the piston for cleaning/inspection/etc.