Garrow Firearms Development (GFD) manufactures the GFD AR-15 .17HMR gas-operated semi-auto upper. This unique rimfire upper is designed to safely and reliably shoot the .17HMR semi-auto in an AR-15 configuration.

Part of the unique design of the GFD upper is its locking bolt. The pre-hardened tool steel is made with precise tolerances and has minimal moving parts for reliability. The GFD Uppers are designed to leave the bolt locked until the bullet passes the gas port in the barrel, releasing a small volume of gas to unlock the bolt when the pressure reaches a safe level. The built-in safety is that the firing pin can’t contact the rimfire round unless the bolt is locked in place. The bolt features two rolling locking balls that lock the bolt in place.

While the GFD upper installs on any mil-spec lower (I installed the review upper on a CMMG MK4 lower), there is another step that you must take before firing. You must remove the lower’s centerfire buffer and spring from the buffer tube. Then, you reinstall the centerfire buffer backwards into the buffer tube. This will act as a space for the GFD’s buffer and spring. Once you’ve installed the GFD buffer and spring with the backwards centerfire buffer, you’re ready to place the GFD upper on the lower.

The upper features an 18-inch stainless steel heavy-contour barrel with internal ramps. The barrel is also threaded for a suppressor or flash hider. I shot the GFD with a Thunder Beast Arms rimfire suppressor. There are a host of .17HMR-approved suppressors at SilencerShop.com.

The upper also comes with two proprietary 10-round magazines. These magazines fed the semi-auto upper reliably during testing. I never experienced a jam or misfeed.

The 18-inch heavy-contour barrel proved extremely accurate, too. I consistently printed 10-shot groups inside of a 1-square-inch target from 50 yards while using a deck railing as a rest.