Mykeal, I was shooting off a rest, so it probably shouldn't have been an issue. But since I was just banging away and having fun when I first got it, sometimes off a rest, sometimes not, I'm not going to say it wasn't an issue then.
Here's my theory. I played around with how much I was compressing the powder last time, and did get a set of high shots. Until recently, I'd been somewhat worried about not getting the ball in far enough. As it was, I now realize, I was compressing the powder a good bit. (I know, a no-no with Triple 7). I varied compression this time around, and while it all makes a kind of sense in retrospect, I was looking at it more in terms of function (recoil, behavior) than what the shots were doing. I should have paid more attention. Looking back, it seems that when I really stuck the ball in there, it shot to the point of aim. When I realized I was being kind of silly and didn't need to press it so far in, I just snugged it up to the powder and it shot to point of aim. That's when I chrono'ed it, and accuracy was so good, that's what I stuck with for the rest of the day. But like I said, I did get one set of high shots there in the middle, and I'm pretty sure that's when those high shots occurred. Given that Triple 7 is reputed to act funny under compression, I'm now thinking that was the difference. I'm embarrassed to say it didn't occur to me at the time. I guess stupid can strike at any time. My money says that was what was happening when I first got the gun.
Gentleman of the Charcoal, I don't mind at all. I get roughly the same sized lead ring using .454s in the Uberti as I do using .451s in my Pietta .44 Navy. I might have to work to get a .454 in the Navy, but you could almost drop a .451 into the Walker. To my eyes, they look adequate (good, round ring), and judging by the chronograph results (1021fps average), I'm pretty certain there is no gas escaping. But its funny you mention it, as I was thinking of trying one size up.
I appreciate everyone's help. I've played around with BP guns off and on for years and always enjoyed it, but the awful clean up kept me from doing it as much as I would have liked. No one had ever told me that oil and blackpowder are a terrible combination. Once the internet wised me up to that, things got a lot more enjoyable. Triple 7 has been a Godsend, so I'm getting into it a lot more. Chucking cleaning brushes into a cordless drill, and an electric toothbrush, to clean out the cylinder and parts helps, too. The flush-out tube from Dixie makes cleaning my Hawken so much easier, I take it out all the time now.