Hi, Mark,
Dr. A. J. Jones, in his book on sporting gun choke performance, pretty much came to the conclusion that choke constriction is the only predictable factor in choke effect. I once spent a lot of time on analysis of various choke patterns. A Charles Hellis game gun with 7" taper length full choke had, surprise, surprise, full choke performance; not better, not poorer.
I think the taper choke Lefever is a red herring. I'm yet to see one in the flesh. Actually, I'm yet to see a choke taper longer than my Hellis guns.
All choke taper is reamed out of a barrel before it is threaded for choke tubes. So, pre-tube choke is no issue.
"Abrupt" could have differing interpertations. A square shouldered diameter change (now, that would be ABRUPT!) probably would not work as a choke. However, a 1/2 inch taper probably would. It is not clear how much shot follows gas dynamics laws; it is clear to me that it does to some extent.
I have had good luck with tubed guns. Go for it if it will make a gun fill your needs.
DDA
