Since no one directly answered your question I will. NO THERE IS NO STANDARD. Furthermore, there is a wide variation from manufacturer to manufacturer. Most T/C inlines tend to be on the tight side. Very close to .499-.500. Most Knights tend to be somewhat larger at about .502 -.504. The spanish rifles I had contact with slugged at .501-.503. And of course the Whites are a little larger at .504-.505. This by the way is only the few rifles that I have been in contact with and as the machining devices wear the barrel sizes vary. Slug your rifle and you will have a lot better chance of getting a bullet to fit properly.