A semiautomatic or fully automatic firearm that includes a barrel, the rear barrel end whereof is in the form of a cartridge chamber into which a cartridge can be inserted, a breech body which is arranged such that it can move in the longitudinal direction between an open position, which releases the cartridge chamber for the reloading of a cartridge, and a closed position which closes the cartridge chamber, wherein the breech body closes the cartridge chamber at the rear in the closed position and is used as a support for the cartridge case of the cartridge, wherein the firearm has a mass simulation body which is in force-fitting engagement with the breech body, in particular a mass simulation body which is rigidly connected to the breech body, the mass simulation body being formed at least in part from a magnetizable or a magnetic material.