Decided that I am going to try casting. I think that I've got the basic process down but have a question about sizing. My understaing is that you need to slug the barrel to determine the bore diameter? What is the relationship between the final bullet size and this diameter? Same size, .001" over, .001" under?

Typically cast bullets are sized .001" over nominal bore diameter. That, however, is not a "cure all". The best way to go is to pour a slug of pure lead, lube it up really well and drive it down the barrel, then mike it. Go .001" over that number for best fit and least amount of leading.

That, and don't cast a bullet that is too hard. It must be soft enough to obturate on firing so it swells and fills the rifling.

Casting is a very rewarding hobby, and I really enjoy it. Welcome to the world of casting!

Best to size to fit the throat. The sizing to the bore will then take care of itself.

Larry Gibson

Larry can you define what sizing to fit the throat is. Rifles have a wide variety of shapes from basically 45° chamfered rifling to .001 over bore parallel throats to long tapered throats.
How do you fit a long tapered throat like a 98 Mauser.

The chamfered rifling like a 30-30 or 45-70 is basically not throat

The long tapered throat is conical and is not a constant diameter.

I have also found that sizing the bullet to the largest diameter that will fit in the chamber gives the best accuracy. I am not sure how to measure that. I have a pretty complete set of sizing dies and I try several dummy rounds with different diameters to find the correct one.

I have found in the guns that that I have tested extensively, that going up to 0.004" to 0.005" over the groove diameter does NOT hurt accuracy. Generally, accuracy actually improved the larger the bullet got (up to the largest that could chamber). The worst case, it improved for a couple of thousandths and then plateaued the rest of the way. The accuracy never went down.

Of course, I have had some mismatches guns that could not shoot regardless of what diameter was used (such as a Ruger that had 0.424" cylinder throats and a 0.430" barrel). Nothing that you can do with them except trade them off or spend a lot of money with a gunsmith.

quote:

I have also found that sizing the bullet to the largest diameter that will fit in the chamber gives the best accuracy. I am not sure how to measure that. I have a pretty complete set of sizing dies and I try several dummy rounds with different diameters to find the correct one.

I have found in the guns that that I have tested extensively, that going up to 0.004" to 0.005" over the groove diameter does NOT hurt accuracy. Generally, accuracy actually improved the larger the bullet got (up to the largest that could chamber). The worst case, it improved for a couple of thousandths and then plateaued the rest of the way. The accuracy never went down.

Of course, I have had some mismatches guns that could not shoot regardless of what diameter was used (such as a Ruger that had 0.424" cylinder throats and a 0.430" barrel). Nothing that you can do with them except trade them off or spend a lot of money with a gunsmith.

HOOORAH, a smart shooter at last!
Only one thing though, it doesn't cost much to get throats reamed and make the gun a tack driver. If I remember, Cylindersmith only charges about $35. I have the tools for the .45 and can do it even cheaper.
But you can also lap the throats yourself.