Just learned I won a charity raffle and am getting a Bushmaster XM-15 QRC. Excited about winning and have been doing some research on the rifle. I'm not a fan of the gas block and would like to switch it out for something low profile and a BCM KMR. It seems like such a basic question, but I can't find anywhere on the net where it says what length gas system it uses. QRC stands for quick response carbine but the system appears to be midlength? Anybody know? Assuming the gas block is .625 for the lightweight barrel profile.

Carbine gas.  Not sure about gas block profile or dimensions.

I think its actually an ORC which is optics ready carbine. If so it's a 7" carbine gas. I would really think twice about dropping too much coin on that upper, keep it as a beater or sell the upper and throw a 16" BCM mid upper on it.

JMO

They sell a QRC.  Cannot find any info on the barrel diameter at the gas block seat.   Personally, I'd use a cut off wheel and remove the sling attachment and bayo lug and then put a free float handguard on it.

Looks like it's essentially an ORC with a pencil barrel. IIRC Bushmaster uses .625 gas blocks on their pencil barrels, it looks that way from the picture at least.

However I'll echo balloo's advice to shave the FSB, I've seen several KMR builds that have them.

Thanks guys. The inner diameter on the BCM KMR is 1.3inches, so I will measure when I take possession to see how much to trim. I'm only dropping coin on the KMR at this point. Got enough parts and pieces laying around to do some extra mods without investing much more. Figure I might rattlecan this one too.

I wouldn't concern yourself with the "anti" bushmaster comment(s)

Absolutely no reason not to put a kmr and whatever else you want on it

There's no need to measure anything when cutting the FSB down. Just Google pics of shaved FSBs and do it like that. I believe the top is already shaved so you would just need to hack off the bayonet lug/sling mount and smooth it out. I usually just touch them up with flat black high temp paint, exhaust system paint or barbeque paint works well