No.4 MkI barrel to channel contact when trigger guard tightened down.

[Roger Payne]

There are some ex serving armourers on this forum, but until one of them decides to chip in, I have a suggestion that you might want to look at. A long time ago I encountered a similar problem, & after a few words of advice from PL I resisted the temptation to open out the barrel channel towards the muzzle on the side that it was binding, & had a look at the routing out of the forend at the reinforce of the barrel end (chamber) ON THE OPPOSITE SIDE. If you think about it it is blindingly obvious (mind you, it wasn't to me until I'd asked!), that a high spot here will cause the barrel to push over to the opposite side progressively as you go from breech to muzzle. I'm not saying it must be this problem, but it could be, & is probably worth investigating. And the solution is simple - carefully take down the high spot a little meat at a time, until the barrel sits centrally in the barrel channel throughout its entire length, with the screws correctly tightened.