I really like my HW45 (AKA Beeman P1). I once bought a replacement breech seal after shooting some crossman 7.9's in it. They were very hard and would not seat all the way in. I though they were dragging across the seal so I stopped using them. The JSB 8.4's are so much better seating and accuracy wise.
Anyways, I didn't need to replace the seal, but it is just an oring. It looks like it would just pop into place and stay there since only a little bit of it is above the breech. Since the HW45/P1's piston goes backwards the recoil is rather unique. It kind of jumps forward. They are also quite accurate. Velocity is very consistent around 527 fps, so 30 or 40 yard tin can shots are very doable. I usually research and buy carefully and this is one airgun I won't be parting with.