Upper

An AR upper receiver is not a firearm, which means it ships straight to your door with no transfer and no paperwork. A complete upper includes the barrel, handguard, gas system and bolt carrier group, so a second one turns an existing lower into a different rifle in about ten seconds.

That is the most cost-effective way to own several rifles: one serialised lower, several uppers in different barrel lengths and chamberings. Check gas system length against barrel length — carbine, mid-length and rifle gas systems are tuned to specific barrels, and a mismatch causes over- or under-gassing that shows up as erratic ejection. Note that putting a short upper on a rifle lower has legal consequences worth understanding first.