You probably won't get a good reading from a rim-fire bullet as they usually run about .222 to .2225. They depend on a cupped base and thin section of lead at the base for gas seal, and inertia and pressure obturation to grooves for accuracy. Best was to slug bore without using a cast bullet, is to get some fish sinkers slightly larger than the .22 bore and then run it through the bore. Like the other poster stated, if made prior to 1950, it may have a .223 bore, if after, probably a .224. It would behoove you to try both to see what shot better. I had a Ruger 77/22 Hornet that shot the .223's better, but was still a stinker. I traded for a 77/22 KBZ with heavy barrel- has much better chamber, but I still thing I'm going to send it to CPC Gunsmithing for the K-Hornet conversion and bolt tightening. I get some really great velocities, but the throat is really long and rough, although the body section is nice and tight, -accuracy is an off and on thing, usually better with deep seated bullets. A really good K-Hornet rework would give me near .221 Fireball velocities and better accuracy to boot.
Try the Hornady .223's and .224's and see which shoots better.
