Hi Lewis,

Much the same as has been said by Hsing-ee. Try the easy stuff first, check for any looseness at the barrel mounts and make sure that the barrel band is not too tight. Then take the silencer off and test. If that does not sort it give the barrel a clean and inspect the crown for damage. This can be done with a cotton bud by inserting it into the muzzle and slowly pulling it out, any burrs will catch a few fibres.

After that you really need to put it over a chrono to check the shot to shot consistency. If you were nearer you could borrow mine.

With the valve screw issue, it would be silly to start on the internals without trying the above first.

Good luck, I hope you get it sorted

Cheers, Mark