I'm modding one right now and just changed out the valve/piston for a flat top set. When I pulled the stock set out, I was amazed that mine was firing as well as it did out of the box. O-ring had large chunks ripped out of it and the whole thing looked like my 1 year old daughter had been chewing on it.
IMO, I would exchange for a new one if it won't group from a bench rest.
Or, if you feel like tinkering, start modding it... by the time you are done, you will have probably replaced the part that is causing your grouping issue anyway.