Semiautomatic vertical feed build kit for the STEN Mk3. The bolt in this kit is machined to allow feeding from a vertically mounted magazine housing similar to the MP-40 and the MP-3008. This is not a modified standard semi auto bolt but a specially machined bolt. This closed bolt system works extremely well and eliminates the problems of kits offered by other companies. You will need this kit, one of our tubes (MK3SAT), and an original Mk3 parts kit in order to build a legal semi automatic firearm. This kit includes the following high quality American-made parts; a semi-auto bolt, cocking handle, standard AR-15 firing pin, roll pin, firing pin spring, bolt stop, guide rods, rear barrel bushing (reduced diameter), rod retainer, hammer, recoil springs, and assembly instructions. Two small parts need to be welded together. This design has been classified by the ATF as a firearm and not a machinegun. 922r compliant.