This is the spring that ships with all Zero Kick Hammers. It is designed for the use with the Zero Kick hammer and can be used with stock type hammers as well. It has the stiffest wire that you will find in the world. If you are looing to increase your max velocity, this is the spring to get.

Extra Stiff Spring for Max Velocity
Can be used with any TechT Zero Kick Hammer
Same Spring that comes with all TechT Zero Kick Hammer
Its a good idea to have an extra spring on hand.
This works with:
Tippmann Cronus
Tippmann FT-12
Tippmann A5
Tippmann 98
Project Salvo
Alpha Black
BT paintball markers
Valken SW-1