It's always worth remembering that there are sooooooooooooooo many variables in the world of 9mm that we, in our infinite wisdom, TELL the designers that our GS requirement for the product, in this case a silenced SMG, is XYZ and the finished product must operate 100% of the time, silently with UKstandard 9mm Mk2z ammunition. At the start of the project any non standard ammunition was not even a point open to discussion.
I seem to remember somewhere in the back of my brain that the commercial Mk5 Sterling had a hard buffer ring seated into the back of the return spring cap. There wasn't one in our L43's but we needed one when we fired off a load of captured Iraqi 9mm made by Gevelot