Twelve shots and ten of them hit the paper. There seems to be a line above which they will not go, but they don't have any trouble falling way below it. Someone suggested pre-measured charges. To see if I had a measure problem I dropped the load from 80 to 70 grains and the top line came down about one inch. Probably means I'm throwing out unburnt powder.

I loosened the wedges by rinsing out their mortices with IPA. They came altogether too loose so I had to bed the barrel with brown paper. When they were loose the barrel would slide slightly fore and aft so it isn't a thermal problem.

I added a bolt to stop the cheater foresight moving, seemed to help.

New theory... The left to right error is me being wobbly, the up down is a dramatic change in velocity. I think I need to buy a chronoscope.

Here's a silly movie (1.7Mb)