I had a Hornet that was rechambered to 218 and it was lots of fun--easy to find a good load and plenty powerful for PDs--40 gn Vmax bullets chronied 2800fps using 4227 powder. Sold it to a guy who wanted it way more than I did, judging by what he offered and paid me for it. I've often thought that if I were to do it again I'd go with a Fireball instead of the BEE, especially since I now have BEEs in a Marlin M94 and Ruger #1. Another point, my converted hornet needed shimming after several thousand warm loads.
I've also considered starting with a rimfire barrel for a rechamber project thinking I'd be able to buy a whole new rifle cheaper than I could buy a Hornet barrel. I still haven't made up my mind on this--here's a few reasons I still can't decide if it's a good idea or not. I'm thinking about the 22 magnum ultra barrel.
I'm at least as concerned about the twist of the rimfire barrel as the bore--if it's tight, like 222 or 221, you'd just use less powder to get the same pressure since the bullets will swage down the extra couple of thousandths and still fly straight--the 1-in-16 twist is potentially a bigger factor--you may be limited to the lighter bullets(under 50 grains). I know my bee, which I never knew what twist it was, got the best performance and accuracy with 40 grain bullets. My current Hornet and 22 Jet are both doing their best with the 35Vmax bullets, I haven't checked but their twist rate should be 1-in-12.
The 221 case is probably more efficient, and a more inherently accurate design--judging strictly by the success the PPC and BR-style cases are having in CF benchrest competition. It would be a great choice if the efficiency, extra case capacity(and muzzle blast) can be realized.
It could also be true that the slower twist would stabilize 50-55 grain bullets just fine, the only way to know is to try it and find out, you might get lucky.
