I know what you're talking about. I have a BSA Meteor that works as you describe. The Mercury and Airsporter have a threaded alloy casting that comprises the end plug and trigger block. It's a beautiful thing, but somewhat vexing when trying to use a spring compressor.
Turns out I was able to break it free using a 36" x 1/2" steel bar inserted in the 1/2" hole in the casting. These guns are quite the challenge.
I think we're all Bozos on this bus.