The most interesting powder for the .410 is Alliant 300-MP. I loaded shells with 1300 fps with pressures in the <7,000 psi range. It even patterns well and does not burn up the case mouth.
On the Mec bridging shot I heard two "cures". One was to drill a small hole in the drop tube to allow air to escape under the dropping shot. The other was to take a three corner file and put a notch on the end of the drop tube to allow air to escape. The claim was that the air was being trapped under the shot, which slowed it down and it would then bridge. I wonder if it's not just a sudden taper with too much roughness on the inside of the drop tube.
Don't know if either was a cure but I tried the notch approach. I've polished the inside of the tube to make it as smooth as glass and put a small notch on the end of the drop tube. After that I have not had any bridging except when I tried recovered shot. That's stuff not worth using in small bores to me.
