I have an old ruger #3 in hornet. How do I tell if it is .223 or .224. thanks.

I suspect it's .224" groove diameter but it does not matter a lot. Just load up .224" bullets and work up a load.

Before I gave up the Hornet I found that the lighter loads at lower pressures were more accurate.

Pre WWII Hornets were .223", post war thru present are .224". Give Ruger a call, I'm sure they'd tell you. That aside, I have a pre war Savage Hornet and I've been using the .224" bullets with very good results.