I have been thinking about modernizing my Bushmaster xm15 e2s which is in the A1 configuriation. I was going to get a flat top upper reciever, (complete upper is out of my budget) along with MOE funiture, and possibly an Eotech 512. My question is will the gas block have to be removed to install the new reciever since the barrel must come out? I have heard the A1 gas blocks are attached differently and cant be removed without some gunsmithing. Is this build worth the time and money and possible headache? or should I trade it in and use the $800 build budget i have plus whatever i get for the old rifle and buy an M&P 15 flat top and modify it with whatever funds are left? Any of you knowledgeable AR guys out there I need some info here, having dealt with mainly handguns I am AR ignorant.