SRY- missed this. Well done.

"a Ballistic Products plastic 10 ga gas seal for the ball to sit"

I also think you need more powder. My WW Greener's book lists 10 drams, ie: 275gr. powder and a 677gr. ball. Since a pure lead 10 bore ball weighs 700gr., I assume this is a lead/tin mix, which is softer than WW, btw.

I really do think you should increase your charge of GOEX 2F to about 7 drams (191gr.) - or even that out to 200gr.

If 200gr. is too much for you, then at least try 150gr. 2F.That is only 5 1/2 drams, just over 1/2 a normal charge.

That little load you list will likely not develop enough pressure to obturate pure lead.

2 1/2drams (68gr.) is a light load for a 20 bore.