Re: NEED HELP WITH A BUILD
Well...first you need to decide which platform you want to run: Armalite vs. DPMS/KAC/POF/etc.
The primary difference between the platforms is the magazine. Armalite uses their own mag setup and currently, there is no aftermarket source for their mags. The mag cost for the Armalite pattern is a little more expensive than the DPMS platform, but the Armalite mags are 100% reliable and built to last a long time even with field use/abuse.
The DPMS pattern can use any number of mags, including DPMS factory, KAC factory, C-Products and Magpul LR20 PMAGs.
The other thing to be aware of is that only some parts/accessories are interchangeable between the platforms while others are proprietary to each platform. For example, barrels/extensions (with some argument) and handguards differ with both platforms.
Once you have chosen a platform, folks here can easily recommend sources for components, including receivers (U/L), barrels, BCG's, handguards/free-float tubes, triggers, etc.
Both are excellent semi-auto platforms to build on (I have used both extensively for personal builds and rifles I have put together for friends/colleagues)...just decide which you prefer.