I don't think it's possible with the recon stock. I was reading on another forum that I think doesn't like its name mentioned here, that rich from mich has one too and said it's difficult or something. I was interested since I have one. I emailed him about tuning it and he said it might put out 600fps after. I have been looking at the gamo nitro, but don't think it's needed right now. This recon can take squirrels at 30ft and is silent but deadly.
Forgot to mention the tune would be more than the gun is worth, so I'd be better off just buying the nitro 17, which is similar to the old gamo 850
I found a bit of it; reply to if recon stock will fit a CFX
"it will NOT, they did away with the rollers on the cocking lever. They now have a button imbeded in the stock under the cocking lever, no provisions for rollers in the stock."