There are a few exceptions (MP5), but, most sub guns fire from an open bolt. The primary reason for the bulge is attributed to those guns being blowback guns. As to Junk I've used an Ingram for the purpose they were designed for. They are perfect for that application. The last gun I want in battle field environment. The first gun I want in a small room with a bunch of bad guys. The ultimate Industrial Ed project was the M3 (grease gun-only the barrel and the FCG was machined) a lazy 450rds/min. They fire from a big massive open bolt witha long stroke. They won't eat you out of house and home either. These were designed to be thrown away. Even the incredibly designed and machined Thompson Submachinegun Model 1923 was an Open Bolt, Blow Back design.