Yes the reports I have seen have shot fine with the adapter fitted, until a silencer is added. Then the grouping can go awry.
The barrel itself must be ok as it groups fine without a silencer, silencers tested were decent so the only real conclusion is the adapters, or at least some of them must be faulty.
I would avoid the £6 adapters and start with the Ronnie Sunshine ones at £11, or for under £20 you can get a nice custom steel one.
Or you could get a Matt Winser MWSS200 push on silencer.