There's also the "old" prowin mechbox shells...and the "new" cnc'd one where the back end is unscrewed and the spring removed (sort of like the 249 setup). (can't vouch for the new one...I've never used it)
There's the CTW clone of the PTW as well.
IMO, the quick change CA/clone 249 and PTW quick changes rule. Not necessarily because of how well they quick change a spring, but because of how well the entire setup/gun works (spring change or not).
I know there are big ICS fans out there...but I'm not one of them. Just my own opinion.
So far as "quick power level changes"...the fastest is to have an indoor gun and an outdoor gun good to go.
Obviously time and expense. Because you have to reset your hopup anyways (and maybe rezero a bit) having two full lowers setup for the same upper is another option.
Since the whole point of quick change springs is to alter the FPS...there are other ways to do it:
- velocity reducer (universally frowned upon as not trustworthy)
- alter inner barrel length...I had a setup where my CQB barrel length shot 340fps and if I swapped in my "outdoor" hopup/longer-inner-barrel it shot 385fps. Just split the gun, pull out the hopup/barrel combo and drop in the other one. For the longer inner I'd put a silencer or barrel extension on the gun.
- there's also that adjustable piston head thing...that "leaks" more or less air to set the velocity. I've never used that so no comment. Concept is sound though for most setups