Not sure how to answer this, we have three AR15's.
CAR15; cuz I have always wanted one so I buiilt one from an Anderson lower, DPMS lower parts kit, A1 upper, surplus 11.5 in pencil 1/12 twist barrel with welded BM fake moderator to be legal. Surplus A1 front sight tower/post, surplus Colt CAR forend, grip and stock. Usually only use GI surplus 20 round mags.
Super light weight M4ish: This is the wife's, 16in 1/9 twist Del-Ton pencil barrel to Anderson forged upper, Oly Arms lower with DMPS lower parts except for MagPull triger guard for wearing gloves, CAR forend, grip and stock (she liked how my CAR felt), She choose an Aimpoint PRO optic and Ma-Tech BUIS.
Then my M4 made to duplicate an issued M4; This one started off life as a factory Del-ton sporter. This only thing left of the original is the lower and forged upper. It carries a 14.5 in chrome-lined BM mil-LEO spec barrel with 2 in welded flash hider to be legal. Surplus side saddle sling swivel, surplus M4 front sight tower, surplus Knights quad rail kit, Surefire 962 with xm07 switch I picked up off a Marine. Trijicon TA01 ACOG with Ma-Tech BUIS, also have Colt M4 carry handle. Colt FA BCG, DPMS lower parts with Magpul trigger guard, Badger tach latch, CAR stock, A1 grip. This took me a while to assemble as I took my time to find as many authentic USGI parts as I could. Lots of fun bartering, trading, and dealing at gun shows, surplus shops, and various other sources. It's a touch heavy, but is my primary, goto carbine. I would like to swap the upper/lower with marked Colt M4 receivers for authenticities sake but am not willing to pay what folks are asking just for special scroll marks. Maybe someday I will.
The Army never should have sent me to Armorer's school. LOL, these things tend to never end with all the parts that are out there.