Battery
 

I just received 2 new Intellect 8.4V 1200 MAH stick batteries and one of the two is doing something very strange. I fully charged it but when I put it in the gun just don't shot, motor try to run but the battery as not enough power. I took a reading with a voltmeter and it read 10.20V fully charged.

When I pull the trigger 4-5 time it finally shot.

Your gun is probable not assembled right. That battery is slightly weaker, but enough to expose the problem.

The gun is working normally with the other battery. Once it start firing it fire normally but it's hard too start.