If Stub came down ballistic, it probably means either the nose cone is too tight or the motor is too loose. Was the motor still in the rocket? When you build another one, try blowing through the motor mount and see if you can blow the nose cone out. And then make sure the motor is well taped in. Some folks put a wrap of tape around the motor to make sure it is tight in the body tube - and then some of those have trouble getting it back out again. My preferred method, if there's some of the motor mount tube exposed, is to run some tape from the motor to the mount tube, so that you're actually taping the motor in place.
When a C6-0 burns out, it sends bits of burning propellant forward, some of which will find their way into the nozzle of the 2nd stage and ignite it. Provided, that is, that the 2nd stage is still there. The C6-0 also produces significant forward gas pressure even though it has no official ejection charge, which can cause the 2nd stage to separate without ejecting. To avoid this, make the booster a little longer so there's a gap between the 1st and 2nd stage motors, then drill 2 or 3 small holes around this gap. That will vent off the gas, so the 2nd stage won't go anywhere before it can ignite. I have successfully staged an A3-4T from a C6-0 using this method. (Mind you, I never saw the upper stage again!)
