Update from just getting one for the GPR.
1. It is designed/works best with the lock off. (Since I pull the lock to scrub it out w/ a toothbrush, that's fine w/ me)
2. It now comes w/ a little knurled ring next to the O-ring itself so as to snug that O-ring
forward and against the barrel. Snug it forward before tightening the bolt portion.
3. Slotted vent-hole liners are no problem. The O-ring easily covers the slot.
4. Mine works fine even with a [very slight] cant due to the stock/wood on the opposite side. Filing off a bit of the that metal to seat lower is a super idea.
5. Bottom Line: It works well -- especially with that dratted patent breech design which is otherwise a bear to fully clean out each session.