there is no spring as such, the small silver disc under the plastic film is cupped and acts as the spring, make sure to remove all the plastic indent catching pieces on the sides to enable easier disassembly.If you want to do some testing , assemble without button and small disc, and fire it by joining contact points ,Aye I don’t know if something has fallen out of it, but once it was apart there was no way to put it back together with any spring in the button at all. This is different from the fault my other one has, which is definitely electronic.
Disappointing, two separate faults that cause autofires.
If it auto fires without disc in place then problem is else where,maybe try a piece of wooden dowel with a small flat head nail in end or a small piece of aluminium foil glued to end of dowel