Have I got a doozy for you guys this time! Jonathan Ferguson at the Royal Armouries did a video on the
Kretz Rifle, a Pattern 13 modified to use a Mannlicher straight-pull bolt, then further modified to be an automatic rifle (note in that era a machine rifle did automatic fire and an automatic rifle did semi-automatic fire).
The mechanism is a wonderfully bizarre blow-forward chain-driven repeater. When the barrel shroud is pulled forward by gas pressure, it pulls a chain on the left side of the rifle that's connected to a pulley on the left side of the butt. That winds up a spring and also rotates a pulley on the right side of the butt. When the gas pressure declines, the spring returns to its normal state, pulling the left chain back and also rotating the right pulley, which now pulls back on the right chain that's attached to the bolt, opening the bolt and ejecting the spent cartridge, and disengaging from the left pulley. That also winds up a second spring attached to the right pulley, which releases when the trigger is released, driving the right chain in the opposite direction and closing the bolt on a fresh round.
It's specifically called out for being excessively heavy, and I'm trying to find out just how much weight that chain drive added to the rifle. It's a terrible design that couldn't work as built and wouldn't work well even if the fundamental flaws were corrected, but I can't help but love the Rube Goldberg-esque effort that went into making this armorer's nightmare.