I have a Bushmaster XM15-E2S and would like to eliminate the front sight and add a set of extended rails. This gun has the fixed flash hider, I'm not sure if that effects my choices or not. I would simply like to shave off the sight portion of the original gas block. Does anyone make a rail system that fits the bill and that I could install myself? Thanks.

Would this work?

http://www.bravocompanyusa.com/Daniel-Defense-Omega-Rail-Mide-Length-Free-Float-p/dd%2010002.htm

Is your flash hider pinned?

If your rifle has a delta ring and the standard barrel nut, then yes the Omega will work.  You just have to cut the sight tower and remove the handguard cap (harder to do if your flash hider is pinned) if the rail is longer than it.  You could even use the rifle length if you want even more rail space.

You could also look at the Troy TRX ALPHA rail, Samson Evo and Rainier Arms, Centurion Arms C4 rails, they use the standard barrel nut, but you would have to cut off the delta ring..

Yes, the flash hider is pinned. I believe the barrel is 11.5 inches long while the flash hider is 5.5 inches long. How much accuracy / velocity am I losing with that barrel over a standard 16 inch barrel?

I'm wondering if I should just buy a new barreled upper that already has rails / BUIS etc. Thanks.