With a [k26] I'm not sure, but on all mine I run Home Depot ever built springs and it's not necessary. The stock catch has really good geometry so you don't need a super beefy spring, the OEM one works great. K26s are a little bit stronger but I wouldn't be too worried if it were me.
No, the [k26] is about the strength of an ace49 which requires a better catch spring. The stock catch can only handle about 8kg max. [k26] is 12kg. So yes, the stock catch spring must be upgraded or the blaster will not work with [k26]. For some more info on fire strikes and [k26], just watch Captain Xavier's video that has the fire strike in it (episode 4?).
Edit: I was just on Amazon, and OMW does in fact sell a Firestrike main upgrade spring. Not the kit, the spring. It is listed as for the strongarm,firestrike, rough cut, crossfire, etc.
Edited by Bubba Longshot, 30 April 2017 - 03:07 PM.
"The only thing that sucks about Nerf modification is the anxiety that something will break... That's why we reinforce a whole lot!" -Me