Everyone is correct. It's gonna depend on age and manufacturer. Most where .222, some .223 but not set in stone.
Best advice is cast or the slug the bore, that's what the barrel is not discrepancies.
That's part of the reason it's defunkt. A Non std bore, got a bad rap from being chambered in a revolver and mediocre performance. Now It's nestalgic, an oddity part of the handi lineage.
I had a contender Jet and shot .223 bullets well enough but the cases where bothersome to make. I was offered more than I paid so I sold it.
The 222R is kinda neat too. But on a bastard bore kinda sours it. .223 dia bullets are available, I haven't seen .222 but truth be told haven't looked either. Yes the 1:16 will limit you to lighter bullets but so does the small case cap. But remember you gonna be limited simply because of the diameter and what's available.
CW