Sorry, you are wrong. The Brits in question had a sling/harness system that allowed them to press out the MP5k against resistance and retract it to its original position. They normally fired in semi-auto mode but were very capable in FA also.
It is obvious that it would be easier to fire any weapon w/ a buttstock.
What you are missing is that there are people w/ skills that exceed your limitations and experience.
mike r