Yes, a separate design. Not the first or the last gas piston operated automatic rifle. A different design means very little in firearms development. Today we are inundated with what are essentially Glock clones (striker fired, polymer framed, double-stack magazine semi-autos) with little real innovation (the Sig P320's modular FCU design being an exception). Also, most semi-auto pistol designs borrow their operating mechanism from the Browning Hi-Power of the 1930s. Others are simply evolutions of the 1908 patented M1911 design or the CZ75 design. There is little real innovation.
The big innovation of the STG-44 was the use of an intermediate cartridge in a select-fire, magazine fed weapon. The STG-44 used a gas piston system (so did a bunch of other stuff at the time). The AK is just an evolution of that same design. Is that bad? Nope. There are only so many ways to skin a cat and of those, some work objectively better than others.
As for other elements that were copies of previous designs, did you know Mauser sued Springfield Armory for copying its action in the M1903 Springfield rifles? Did you know Glock sued Smith & Wesson for copying its design with the Sigma pistols and that for years S&W paid Glock a royalty to use the design after the suit?
Did you know Taurus made Beretta clones were made on original Beretta equipment that had worn out to a point Beretta did not want to continue using them? Beretta also moved production of its Couger line of pistols to Stoeger (Beretta owned).
Beretta owns Benelli, Stoeger, and Franchi and at one time was in a partnership with an Italian firm called Breda for manufacture of inertia driven shotguns. The inertia driven shotgun action was originally developed by Breda and shared with Benelli during this partnership. Breda and Benelli both retained the right of manufacture after the split. The inertia shotgun action is used by Franchi, Stoeger, and Benelli but not by Beretta. Breda uses it though.
The gun industry and history of gun design, company ownership and manufacture is fascinating and pretty convoluted.
(post is archived)