I was firing my "PDW" (semi) with a Gemtech Raptor and using my new #28 lp. I was having a heck of a time sighting in my z-point and thought maybe the can was to blame. With the can removed, my K was transformed into a single shot repeater. So I guess those locking pieces do make a difference after all.
Eventually I learned the problem was with the old junk ammo I was "using up". I swear the group was 3 feet at 25 yards! I got it down to inches with regular 124 and 147gr duty ammo. With the Remington subsonic, all you hear is the action (which is loud in itself) and nothing else.