Bruno
So your mk2 is a slide thumb type?
So the rod on the front sight/latch slides inside the hole in the underlever? This is actually bending the barrel down and giving a 60mm POI shift at 16m?
If it was fine before I suppose the obvious answer is to reduce the thickness of the Vortek seal to 4.1mm.
This lip ...
He's on about the front of the compression tube. The bit where you pop the new breech seal in is the comp tube nose ( I'm calling it the nose ). The nose ( which has the recess for the barrel rear end to go into to seal with breech seal ) screws into the main tube of the comp tube. The 'lip' is between the front edge ( face ) of the main tube and the front edge ( face ) of the nose.
There is debate about whether it's a lip ( suggesting main tube is forward of nose ) or a step ( nose is forward of main tube ).