to build a AR in .243, you would need to use a .308 lower. The .243 round is too long to feed from a .223 magazine. And a .308 magazine is too big to fit a .223 lower. The exception to all this is the Colt SP-901/LE-901 guns.
Brownells has an excellent on-line step by step guide to building both a upper and a lower. Assembling a AR trigger is not that difficult.
Some Colts use the large trigger pins, It was part of Colt's attempts to prevent one from illegally converting one of their semi-autos to full auto by using small pin select fire M-16 parts.