Gun not firing?
 

Is it possible to put in a spring strong enough that a 8.4v 1200Mah mini-battery can't pull it back? If so, would an M120 (systema) spring do that? Any other reasons my gun might just kind of click and try a little bit to pull and fail to actually fire?

Try some one else's battery.

1200 mah is a little weak..

people at my feild use M120s with mini batteries just fine. try another battery and if that dosent work and you feel confident, take a look inside for any broken or burnt parts.

New 8.4v minis...with good cells (i.e. Intellect, GP, Sanyo)...will pull a M120. Even my first battery which was a Sanyo 8.4v 600mah mini would pull one. After a very busy season (hit a game every weekend, sometimes two on a weekend), my minis (8.4v 1400mah Intellect) were really struggling with stiff springs. Downgraded to M110 or M110 springs and they'd work...but M120's were pathetic.

Had the same happen with an old motor. With a new mini, it just wouldn't pull a M120. I put in a 8.4v large (subC) and it cranked. I ended up tossing the motor and it was all good again.

Alright, thanks for the input guys. I think I'll try and weaker spring first (it was a little hotter than I'd like anywho). I'll let ya'll know if something works out.