The mighty popular K26 Spring made popular from McMasterCarr to be used in hundreds of nerf modding upgrades. See Captain Xavier’s Can it Take a K26 for examples.
These have been custom ordered to have squared and ground ends so that it’ll sit flatter in a plunger than the ones from McMaster. The grounded ends means you can actually stand some of them straight up on a flat surface. They are grounded and closed on both ends, so even if you cut one side, you’ll still have a flat surface to work with.
Dimensions are 0.840″ OD x 0.640″ ID x 9.25″ Lg, with 0.080″ Wire Thickness