Like Gurney said, don't mess with it unless you:
a) have a pile of money in case you screw something up
b) have a shed load of time before you play in case you screw something up and
c) have friends who wont alugh at you in case you scrwe something up.
Having said that, cockers aren't actually complicated gun really. Common sense and some research and taking you time and you'll be fine. What are you actually trying to replace? ANS Pnuematic is a bit vague!