I have always had this problem with my .410 Grabber. Aside from the graphite and tapping the tube I have learned to bump the handle smartly at the top on the return stroke. It usually dislodges any shot bridging, but it is a frustrating thing when I forget and it spills all over the place. This move may slightly increase your powder drop, according to what powder you're using, so you might want to check it on the scales a few times to be sure.
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