[ybrik]

locking during semi-fire is caused by improper motor height. Adjust without the spring and piston. You could also remove the whole cylinder assembly so you could see the gears turn while adjusting motor height. By the way, use like 8.4V (or lower if you have) battery when you do this since your motor is not subjected to a load (spring uninstalled). You'll know the optimum height, you'll hear it.