Hey all. I'm new to the forum and this is only my second post, my first being in the Introduction section. I'm currently building my first AR and I've been trying to do my homework and taking my time with it. I have my lower receiver assembled and I had some questions for my upper. Here is my parts list so far. Lower: ~Aim Surplus stripped lower receiver ~Magpul Miad Grip Kit OD Green ~Magpul CTR stock OD Green ~PSA Buffer tube and buffer ~Magpul Polymer Enhanced Trigger Guard OD Green ~Del-Ton LPK (I believe I heard somewhere these are DPMS parts?) Upper: ~Del-Ton Chrome lined M4 Barrel ~YHM Specter Length Freefloat Diamond Forearm ~YHM Low Profile Gas Block ~YHM -28-5C2 Phantom Comp/Flash Hider W/Aggressive End ~Carbine length gas tube So that's what I have so far. My next purchase needs to be an upper receiver. I was thinking a RRA A4 upper but I'm not sure if it would work with an M4 barrel. I know it's a silly question but I just want to make sure. Any advice on this? My next question is about the BCG. I've been looking into a Spike's Nickel Boron BCG. Also I wanted to replace my current trigger and hammer with a Nickel Boron trigger and hammer. Again, any advice? I hear they are good but are they worth the price? Would I be just as well off with a chrome bcg?