Thanks for the suggestions!
I do have a Krytak CRB that I was looking at converting but it seems a bunch of the parts are proprietary so if they break under increased strain it will be difficult to fix. Might just be easier to start with something more upgradable even if it is painful to spend more money?
Out of the guns that are instock... the CYMA CM.097B seems ok, could I just add in a bigger spring and modify the selector plate?
(only thing that gives me hope for the Krytac is that the US versions ship with a 30k motor and a m120 spring where as the Uk version is downgraded -20k and M100. Could I get a new motor and maybe an m140 spring and get away with it?)