I assume your barrel is free floated Gary, have a look round the chassis front support and see if you can see daylight round it. If the barrel is touching every time you apply pressure to the stock/ grip you will press on the barrel causing this kind of effect.
If it's not free floated, then do it geezer
Unless you are going to wear a gimp jacket and shoot indoor 10m with it
Failing that, as mentioned faulty stabilizer, maybe one spring of 3 missing or jammed up, faulty loading port seal or reg going down.
Also gunged up hammer assembly.
Take your pick dude