damage in the middle almost makes me think you been shooting those PBA pellets or some other ultra light pellet or your breach seal is bad OR the seal isnt fitting well letting the piston slam on its forward stroke...... when you pulled her apart did you just lube it or did you deburr everything too? if you didnt deburr averything there is a good chance you nicked the seal up when you put it all back together...also you said you tarred the spring.... but there was no tar when you pulled it back out..... did you use REAL heavy tar or some molly grease or whatever you had laying around? because heavy tar stick to that spring pretty good and doesnt realy fling off at all...also make sure your barrel is in there tight and the bolt has not loosened.... it could be loose causeing bad lockup with would also be throwing pellets around AND causing you to get a poor breach seal letting the piston slam.... prolly about a dozen other small things to check too but thats what I would start with....
I had that trouble and took mine apart. Everything looked fine. The seal was in good shape but appeared to be hard. Why I thought to think that way I don't know. I had an extra seal that fit and it did seem softer or more pliable. Changed it and now shoots better than ever. Hope this helps ......Forgot to mention that before I took it apart it sounded different once in awhile, like a slam. Mine too gave me pretty good Chrony results before I took it apart. Guess that is why I thought about the seal and checked it. As a slam had to be the seal causing it.I do not use the ultra-light pellets in any of my rifles.