Good point about the liner dia., Zonie & 1/4"X28 would be better- I just wouldn't use a liner IF it wouldn't have 3 threads purchase. Hollowing it out from the inside is a good idea though. I wouldn't wag the drill as it would enlarge the hole onthe pan side - somehthing counterproductive to good ignition. Hollowed from inside will bring the powder to the pan. You don't want a kernal of powder in the hole as that slows ignition.
: In these large bores, pressure are VERY mild, compared to even .54 cal. In the .54 you can get pressures up to 15,000LUP with 150gr. 2F or so, but in the .69's and larger, you'd be hard pressed to get them as high as 8,000LUP. I can guarentee you won't like shooting that one ss it would take upwards of 180 or 200gr. 2F to get it. Using 1F for the .75, the pressures would be even lower. this is why .69's and larger RIFLES shoot so well with paper ctgs. My 14 Bore(.69)English Sporting Rifle would hold 1 1/2" groups @ 100yds. with 165gr. 2F and 480gr. RB in a double wrapp of typing paper. Rip the bottom off, stick it in the muzzle & by the time I got the rammer out, the powder had all drained (seconds) top the breech, ram the paper and ball down together and let her go. No burning paper due to no blow-by and terrific accuracy for 10 in a row. These were especially good for the special weapons hunt for moose as the temps ran right down to -56 several year when we were there. Even Mink oil and Bear's oil got hard & destroyed accuracy, but the paper ctgs. always shot centre. The point being, low pressures are created in large bores. When you get into the shotguns with long shot columns and several wads, the pressure are higher for the same bore due to the inceased drag in the bore, ie: up to 8,000LUP with 4.5drams & 1 1/8oz. or 1 1/4oz shot as per Lyman BPHandbook. With single ball, the pressure is much less even though the weight is the same.
Daryl