This is the famous dirty Harry
cartridge which used cut down .308 Winchester cases
and a .429"
bullet, aka .44 Magnum
, and was fired from an Automag pistol.
The idea behind it was to create an autoloader that could deliver .44 Magnum performance which until then could only be done using a revolver.
Like the Desert Eagle this pistol is gas operated and featured a rotating bolt.
Considered to be a commercial failure, the Automag still has gained its place in the legends of movie guns.
Today, if you want an autoloading .44 Magnum, the Desert Eagle would be the most available choice. Personally, I, would not recommend this practice of firing revolver rounds in autos and vice versa though it is safe and I am sure fun, it's just that more often than not guns like these are a kind of a novelty and are somewhat solutions that are looking for a problem. If you want a real strong kick out of your handgun, try and fire a Biggest Finest Revolver in .45-70
I'd have my wrists insured first though.