Puff of smoke and powder gases coming out of the chamber as Luigi was firing already confirmed to the vast majority of gun-familiar viewers that the weapon is NOT a Welrod or Station Six or anything like it. Manually operated actions are not going to vent gas out of the port upon the round being fired unless the structure somehow catastrophically malfunctioned ie. went kaboom...
ETA: Closed breech rifle and carbine actions venting out the wrong end WAS a minor problem with early breechloaders like the Sharps Model 1859 and it's modern reproductions. In the curious case of the Sharps, small grains of loose powder from the "cigar cutter" action of the breechblock shearing the rear of the paper cartridge, falling into the action's moving parts and then ignited by sparks from a subsequent shot and causing flash of flame uncomfortably close to the shooter's face was one of the issues in originals and the modern repros. Civil War reenactors and skirmishers refer to the more hilarious results of this feature as "Sharps Eyebrows"...