CMMG Banshee Mk4 .22LR Dedicated Upper Group

The CMMG Banshee Mk4 .22LR upper is a complete dedicated rimfire upper group that drops onto any mil-spec AR-15 lower, bringing its own barrel, M-LOK free-float handguard, blowback bolt, and charging handle. It is the recommended host for a forced reset trigger built around .22LR because a purpose-built dedicated upper feeds and extracts more consistently than a drop-in conversion bolt run inside a 5.56 upper. The 4.5-inch variant carries a 1-16 twist barrel with a 1/2x28 threaded muzzle for a rimfire can.

The mechanical detail that matters for a forced reset build is how the .22 bolt cycles. Unlike a centerfire AR where the bolt carrier travels back into the buffer tube and compresses the recoil spring, the dedicated .22LR bolt is straight blowback with its own captured recoil spring inside the carrier; it reciprocates entirely within the upper and never enters the buffer tube. That means the lower's buffer weight and recoil spring rate, the variables you tune on a centerfire FRT, do not drive reset timing here. The timing window is set by the bolt mass, the rimfire ammo's low impulse, and the trigger geometry instead, which is exactly why this is the most sensitive forced reset platform.

CMMG lists the upper at $799.95 MSRP; OpticsPlanet carries the 4.5-inch Black variant at $639.99. CMMG also sells a cheaper drop-in .22LR conversion kit (the Bravo) for shooters who already own a 5.56 upper, but the dedicated upper is the more reliable foundation for a finicky FRT build.