Just an idea but it might work.
One of the annoying things about the '77, '97, TX and LGU is the fiddly loading. So why not put a pop-up loading guide that lifts when the cocking lever is fully back and is pushed out of the way by a spring when the lever goes forward. The '57 has a pop up breech that shows you can have a lift up mechanism powered by the lever. It would also be a useful anti-beartrap device as the piston couldn't go forwards while the loading guide was in place. Alternatively, you could just make it spring powered so the shooter has to push it down to close the lever and move the piston forwards (like the anti-beartrap on a Dianna)

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