AR-15
A modular semi-automatic rifle platform; the most common centerfire sport rifle in the US.
Also: modern sporting rifle, MSR
The AR-15 is a direct-impingement (or piston) semi-automatic rifle originally designed by Eugene Stoner in the late 1950s. Its modular design — separate upper and lower receivers, swappable barrels, free-floating handguards — and a mature accessory ecosystem make it the dominant platform for sport shooting, hunting, and home defense in the US. AR stands for ArmaLite Rifle, not assault rifle.