I had the good fortune to meet Kyle Paaren, proprietor of Paaren Firearms, who specializes in rebuilding classic machineguns – often by rewelding demilitarized receivers. Many of these are brilliant pieces of engineering whose reliability and durability were proven in the mass military conflicts of the twentieth century.
I think the MG42 is the most impressive: It shoots full-power .30-caliber ammunition at a rate exceeding 1100 rounds per minute. It uses a roller-locked bolt mechanism that is unlocked by muzzle pressure (captured in its distinctive muzzle device) pushing back on the barrel itself. Its barrel can be changed in under five seconds.
I was allowed to take photos of some of these recent rebuilds.