Thompson Contenders fire rifle loads, though they are also a single shot breechloader. They are spectacular when firing the hotter loads, too, but the flash isn't blinding or too bad considering it is only a single shot fired. Multiple rounds, as Kaeto pointed out, would be a problem in a gun like the Mauser, though, and not just from the flash. The gun itself had a complex mechanism and buildup from powder residue would cause it to jam a lot. A lot. The Luger was the same way, which is why they stopped making those and switched to cleaner, simpler blowback systems like what is used in the Walther P.38. Also, limp-wristing a heavy gun like that would increase the misfeeds considerably.
Still, if you absolutely, positively, just have to kill everyone in the room as fast as possible then why not go with an old Bushmaster ARM? 5.56 caliber, reliable AR-15 action, bullpup configuration, and fired one-handed with the stock designed to brace against your bicep. You can fire two of the things at once, even, though combat reloads would be problematic. They came equipped with a flash suppressor, and used standard AR type magazines. You could even use a drum magazine (back in the day the single drum was used) for more mayhem. It wasn't just an SBR or AR Pistol but deliberately designed for one handed use and I found it very handy and comfortable to shoot. One of my regrets in life was not buying the one I shot when they were still made, instead I just had to have that CAR-15. sigh