Yeah, I've tuned my Crosman NPSS. First thing I did with mine was install 5/16" brass washers where the breach spacers go. I used a C-clamp to press fit them into the recess that holds the spacers. This tightened up the barrel to the breach block substantially and didn't allow for any sideways movement of the barrel at all. I also tightened down the barrel pivot pin/screw so that it holds the barrel tight enough to prevent barrel droop.
Next, I took out the internals using a home-made spring compressor. I replaced the gouged piston seal with new ones that Crosman sent me. I deburred the receiver tube using a deburring tool, removed the lubricant with Goo Gone and polished the inside of the receiver tube/compression chamber with the green dishwashing scrub pad wrapped around round stick. Wiped down the inside of the receiver tube with a rag. Then I checked the piston for rough edges on the inside where the gas ram nestles in when compressed and there were none, so I lubed up the piston and new piston seal (on its side only) with JM's moly paste. I carefully refitted the new seal into the receiver tube making sure that it was going in without getting caught on any of the edges of the receiver tube openings. Once that was in, I lubed up the gas ram body and inserted it up into the piston and all the way up the compression chamber. Once that was in, I reinstalled the trigger group and everything else in the reverse order of disassembly. Put red loktite on the main bolt that screws into the receiver tube backplate (this bolt is also the one that the stock screw screws into). Put blue loktite on the 3 stock screws, let it set up for 24 hours and then test fired it. The first few shots, it dieseled, but after that it quieted down and was shooting very good groups.
THEN, after I had it shooting 1.5" groups at 65 yards with CPHPs, the gas ram decides to go bad. So...out with the gas ram and in with a replacement from Crosman (under warranty). I haven't shot it a lot since I fixed it, because I've been working on upgrading my Gamo CFXes (all 3 of them a .177 with synthetic stock, a .22 with synthetic stock and a .22 Royal) with JM seals and Theoben gas rams. They're done now and I am looking forward to spending a day with the last CFX I tuned and the NPSS I fixed, putting them through the chrony and seeing what pellets they like best. So far, the pellets the NPSS likes best are the CPHPs.
Hope this helps.
