The 5/8" diameter, 0.072" wire spring is entirely too stiff for the flap button. I think the button is too hard to press with this spring. As the spring gets longer it's easier to compress for a given stroke, but it needs to be the right length to keep the total amount of compression force down. So there is a trade-off. The plans call for a 5# spring - and this spring is around 15# when it starts 5" long and is compressed to around 3" when in the handle working range. Cutting the spring to 4" gave a little less force, maybe 10#.
So, I switched to the 1/2" diameter, 0.054" wire spring. This is much better. I feel the button force is about right, and there seems to be plenty of force on the locking pin.
