I'll bet that being plastic, the velocities are crazy and the bullets are frangible. Maybe perfect for air marshals?
I grabbed some .40 S&W brass and some spare LEGO heads. Sure enough they seat into the casing. I even loaded them in my Glock 22 magazine and fed the mag into my Glock 35. Just to see what would happen, I released the slide and the Lego cartridge chambered without any deformation of the Lego head.

