what mk is it ?
First thing for me would be barrel cleaned.
second would be to check what the muzzle fit is like.
If you undo the wind indicator ring and slip the assembly of,see if it slips back on without moveing the barrel.
If barrel has to be moved to get it to fit then he may need the barrel floated.
May help to say what mk it is ,as they all have there own problems.
Mine is a mk2.
hope this helps