Hi Hawkeye,
I've repaired a couple of these for a collector pal; they work fine with normal 5.5mm pellets, they push in and "size" down easily with a pellet pusher, you can make one with a metal pin fixed in a piece of wood. This design can be a pain to seal, I notice the one at Protek has low power. I have had to faff about shimming the breech block to be a close fit to the barrel as it relies on a metal to metal fit only to keep air in. They do come up for sale but I'd recommend seeing what you're buying and making sure it actually works!

torrens

ps I'd reckon 7 or 8ft lb is more likely.....