Greetings, I am a American rifle named, the AR-15. My barrel length is 16 inches, my caliber is the 5.56x45mm NATO and my cartridge is 5.56x45mm. And my service started in 1964 a long one, might I say. I have two similar siblings, my sister, M4A1 and my brother, M16A1. I shoot one shot per trigger pull. While the M4 is automatic, and the M16 has a Burst function.