Guy is off to a great start.
I might suggest a couple of areas he might want to try to incorporate.
Put the magazine in the grip. That will eliminate the long tube hanging off to the side which is more cumbersome and subject to damage.
Instead of needing to cock the hammer each time, have a component that is moved to the rear under recoil. I bet it might also be adapted to pickup a round from the magazine.
Not sure about how to return the moving component back into a firing position but a spring might be considered.
I also guess some method of locking the whole thing down while firing would be necessary as would some way to remove the spent case.
Sounds like a lot of work, but someone talented might be able to solve those problems.