Got a well worn 600 3 days ago that had been inactive for years. Here is the saga of my attempt to get it to shoot:
I oiled the gas cylinder up to get the seals lubed, put some oil on the piercing pin and seal, then installed a CO2 cartridge. It leaked a bit from the piercing pin cap, but not too bad, so I dry fired it to get the Pelloil distributed somewhat. Took it out to test fire it after loading pellets. It fired OK, but needed to be hand cocked after every shot and the pellets were not firing through the barrel. It just kept flattening the new pellet's noses, so I cleared the flattened ones out and fired again, same result, no pellet was fired and the one in the chamber had a flat nose. Sooo, I did it once more, hoping to solve the problem. (I now, that is the definition of lunacy.)
The 6th shot resulted in a breech jam, locking the pellet loading arm in the breech. Now I began to think the gun might need work. I had a hard time just getting the loader to move at all. Finally, by holding the cocking slide back, I was able to see a pellet flattened sideways under that loader. Got that out after a couple of minutes with a probe and the breech loader returned to its proper position. That was when I saw another pellet in the end of the barrel next to the breech with a crushed tail. It was jammed into the barrel, no way it could allow a pellet to fire, but it was deformed enough to let the gas by it! Got it out. checked the barrel for any other obstruction and finding none, reloaded with fresh pellets. Both of the pellets that were jamming the action were the old style wadcutters with striated waists, not the smooth modern ones I had loaded, so that was probably why the gun was put up.
Now the gun fires perfectly! Great power, no jams, shoots a pellet every time, and no CO2 leaks. What a relief. I put 30 more pellets through it yesterday now I'm sure it is now ready to go. It has held pressure for 3 days now. These are just wonderful guns, way advanced for the time.