Maccari sells some over sized piston seals. I wonder, if you cut back the face of the seal, this would size it down since the sides taper inward, wouldn't this lessen the hollow spot? We used to have an old tuner that would do this and it just brings the piston dovetail forward to fill the void. The amount you could trim back would be limited by how much too over sized the seal is. Maccari sells several different seals ranging from .995" to something just over an inch. Next time I get an overly tight seal, I will try sanding the face flat with some wet and dry. This would give you a tiny bit more stroke, which was the reason the tuner I mentioned did this. He was always looking for a few fps with the old guns. This may not work on all MarkIII stroke guns because the piston might hit the rear guide flange before the rod latches in the trigger. Ask me how I know this.