The same sort of happened to me upon recieving my CA249, I made the mistake of using a 9.6v initially. However, I seem to remember the reason as to why I did was because the large 8.4v was too hard to get into the stock, in fact I began to split it which prompted me to just slide the 9.6v in.
Anyway I didn't get off more than a hundred or so rounds when the stock CA piston was stripped. I took it to Jeremiah and we found that the last few teeth were totally annihilated. So I asked him to replace the stock CA piston with an ICS aluminum piston I had laying around. After putting the gearbox back together though the stock sector and spur gear started to lose teeth. The sector gear was replaced with one from my CA M15A4 and the spur gear was just one Jeremiah picked out. The next day when I loaded it up and tried to test fire it, I hear yet another grinding sound, I'm thinking the bevel gear has now suffered catastrophic failure.
Now I'm starting to get a little POed, I'm trying to figure out what I can do now to stop this thing from self destructing. (I did not upgrade the spring, it is still using a stock CA spring)