You're not off, by much. I now have four functioning .22HP barrels in my arsenal. I recently slugged all of them, and did chamber casts as well. Dimensions weren't as different between them as I thought they would be. Three have .228 groove diameters, one is .227. Depth of grooves run .003 in the three .228s and .0035" (near as I can tell) in the .227, which equals .222 and .220 bore diameters respectively. Throat diameters tell a different story. The throats all run .230, except the .227 barrel which is .229. Again nothing out of the ordinary, as typically a throat is one or two thousandths bigger than groove diameter anyway. I went to all the aggravation of measuring those barrels with the aim of having a custom mold cut for cast bullets. (To throw a bullet cast from wheelweights at .230", sized to .2295", with a bore riding nose tapered to match the taper of the leade, with a length of .7" to stabilize in a 1-12" twist. Cast bullets should be sized for a nice fit in the throat, not the groove diameter. The molds I have now aren't perfect in that regard.)
I wonder if Horace Gephart was speaking from empirical observation or repeating a theory he heard at the counter of a country gun shop? Who knows.
As I believe I stated once before, a .224 bullet is certainly viable in such a large bore. Is it "bad"? Depends on one's definition of bad. It just won't bump up to seal said bore quick enough to minimize gas cutting as a proper size bullet will. No big deal if not very many shots get fired per year, IMO. It becomes a big deal if the gun is shot a lot. Let's face it, not many of us here shoot them that much, except for me and one certain member in Michigan whose initials are "saddlering" who shall go un-named, and perhaps one or two others.
