Don't do it....been there done it...end up spending a fortune...before you know it you've spent 200 hundred....chambers may have to send the barrel to your local RFD ....unless you "send" one in for "repair"...Seriously though once you've fitted a modded breech,barrel,updated gas valve,adjustable hammer,trigger...compressed air conversion ..etc etc... You'll still only have a modified ratty standing you at £££,s with a crap barrel and fires all over the place with a really steep power curve(co2)... 
I did all the above....then sold it lost a fortune and bought a cracking Airarms s200 multi sho