The threaded barrel on a PMR 30 is for a fancy looking flash suppressor. They recommend against trying to use a silencer. It is a marginally tuned weapon IMO.
The .22 Magnum is a difficult round to make function properly in a semi-auto. That is one of the reasons they do not have multiple barrel lengths for the PMR-30.
A straight blow-back .22 LR is common and easy to make.
I understand that the velocity of the magnum out of the PMR-30 is about 1300 fps.