OK TK's for 350 and less and Prometheus for over - thanks! That makes a big difference on what to look for, as the lengths I need do vary
I am certainly going to replace the plastic hop up in Diablo. And I'm currently researching which pistons are in there as I want metal of some kind - plastic just seems all around like a bad idea.
While I'm in there I'm going to tweak the springs as well - I need to get the m4 around 450 fps for outdoor play and the a second spring for the Diablo to get it to that range if need be for back ups - and - another for the M4 to get it around a solid 350 if the diablo goes and i'm in CQB.
1 hop up - 2 barrels - 3 springs and possibly 2 pistons and I should have some solid performers.
I know there is a pile more to do - but getting things reliable steady and consistent is the first order of business.