Could be serious copper fouling... Have you tried something like JB bore paste?
I had a 452 and it was a bastard to clean as the bore was super tight.... I got some of the Bore paste and she came right again.
Hope that helps.
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Have you looked to see if he's dinged or knocked the suppressor and bent it?
I bought my Marlin HMR and the previous owner said it wouldn't shoot groups. Turns out the bullets were clipping the exit hole on the suppressor on the way out.
New suppressor and a clean. IT is awesome.