The SL-9 by Hoffman Tactical

The SL-9 is a 3D printable AR-9 lower receiver. The lower uses a double-stack 9mm Glock mag and features a last-round bolt hold open (LRBHO).

Based on the V4.8 SL-15 lower, the entire back end of the SL-9 lower rear of the bolt catch is in fact the same as it’s AR-15 counterpart. The forward end of the lower has been heavily modified to accommodate the Glock magazine and the bolt-hold open mechanism. However, the swivel pin plates are the same as on the SL-15, allowing for the same printed cheek plates and, optionally, metal swivel pin plates.

AR-9’s use a fixed ejector, usually made from a steel plate bolted or pinned into the lower. This metal part is not standardized and would add cost and limit access if it were made custom. The The solution was to use a printed ejector. Much testing has shown these to hold up well when printed on the proper filament. Even when printed in PLA Pro, the ejectors hold up very well. The ejector is retained by the upper, eliminating the use of fasteners.

The mag catch is similar to the ejector in that there are no standardized AR-9 mag catches as There are AR-15 catches. The solution is the same: the mag catch is printed. A standard AR-15 mag catch spring is used with the printed ejector to simplify sourcing.

A LRBHO is a challenge to 3D print due to the need for high stiffness and small parts. The solution was to use a hexagonal metal shaft to transfer the torque from the magazine follower at the front to the bolt catch in the rear. A combination of a sliding element and swinging lever were used in the front to eliminate layer adhesion issues. The last feature was to place the rear lever below the main body of the catch, rather than under the standard lifting nub. This greatly reduces friction and has resulted in zero failures to hold open during testing. The metal shaft is easy to obtain from a cheap hex key or aluminum extrusions.

The rest of the lower part is very similar to the SL-15. A rib on the left side works in combination with the buffer band to produce a strong buffer tower. The pistol grip is integrated to improve the strength and ergonomics of the lower.