The 12 bore to 10 bore will be about right as well, with a .045" (or thereabouts) restriction bore to bore. I don't know what the rifling depth or bore size is on the NEF, but assume it's around .729" to .730" which is normal 12 bore. A 10 is supposed to be .774" - hense .044" to .045". Depth per side sould be .0225 average + depth of the grooves, which might be in the .010 to .015" range.

Sounds good to me and very interesting in fact.

Another possibility would be to rifle the chokes of a 12 or 10 bore. Most full chokes have a 40 point (.040") total constriction, giving .020" per side. Rifled in perhaps 70 or 80" for round balls might make a wonderful poor-man's paradox.

I'm anxious to hear of Daves bench-type results opn paper with good round balls.