On Wednesday, the Supreme Court heard oral argument in Garland v. Cargill, which poses the issue of whether a semiautomatic rifle with a bump stock is a machine gun. A machine gun is defined as "any weapon which shoots … automatically more than one shot, without manual reloading, by a single function of the trigger." 26 U.S.C. § 5845(b). If a gun fires automatically, i.e., without any manual manipulation, and it does so with a single function of the trigger – which could be a pull or a push – it's a machine gun.
In the very first two sentences of his opening statement for the government, Brian Fletcher unknowingly explained why a bump stock is not a machine gun. With a bump stock, one "places his trigger finger on the built-in finger ledge and uses his other hand to press the front of the rifle forward. As long as the shooter maintains that steady forward pressure the rifle will fire continuously…." What he didn't say is that if one just pulls the trigger but does not manually continue to push the handguard forward, the gun fires just one shot and stops firing.
A video is worth a thousand pictures. Watch how a machine gun fires. You can hold it with one hand and just pull the trigger, and it fires continuously until the magazine is empty. Now that's automatically by a single function of the trigger.
