Quoted from Zablon:
additionally I get quite a few cannon shots that land above the targets so they don't go down
AC/DC drops, and just Stern drops in general, are notorious for being able to take a hard direct shot and just stand there stiff as a board, and not drop, so I don't think your shots are going above the target, I just think you're seeing Stern drop targets being Stern drop targets. I had this issue with my AC/DC Luci VE, and after a few months of tweaking, I got them to drop 99% of the time (this includes the TNT bank which had the same problem).
First off, I don't think the angled face of the drop targets help them drop at all. I think this prevents airballs pretty well, but if you had a high speed camera, what I think you'd see is a hard fast direct shot hitting the drop square, then the angled face of the target riding the ball upward as the ball moves forward after impact. The drop target can't drop because the angled face is being lifted by the ball. In cases where the ball rebounds away quickly, I think the angled face of the drop target allows the ricocheting ball to simply drop the target back onto the edge it was sitting on. This is why light hits and edge hits seem to drop the targets more effectively than a head on hit. To counter this behavior, I placed .060" shim strips under the rear mounts of the drop target assemblies, which effectively tilted the drop targets back, making the relationship of the angled faces to the playfield much shallower. This helped a lot, but not enough.
I then took all the drop target springs and I unhooked one end of the spring, then reattached the spring to the spring hook about 1/5 to 1/4 of the way down the spring, effectively making the spring stronger. This spring adjustment and the shims got me to almost perfect drop behavior.
Your mileage may vary with these mods, but I just wanted to let you know what worked for me. Note that I didn't come up with all of this. I believe I found these ideas buried in this AC/DC thread, and tried them out with success.
I wish you good luck and a happy new year. AC/DC is brutal. BTW, I get quite a bit of SDTM drains from the bell lane with perfect leveling. I can't help you there.