Steve,
Slug those throats to make sure of the acutal diameter. Using calipers will put you in the ballpark, but are a poor definitive answer to determining actual, precise cylinder throat diameters.
Finally, if they are really that tight, I would highly recommend opening them up,( or having it done) to a .4105". This diameter will allow excellent accuracy with .410" diameter bullets, yet still allow for a slight buildup of carbon and powder residue in the throats and still have ammo chamber easily.
If the throats are under .4095", your accuracy with cast bullets will never be what their potential could be with larger throat dimensions. They will act like an undersize sizing die, giving you an undersize bullet to traverse your bore.
Only slugging will tell for sure, so give us a report once that is accomplished!
Slugging instructions are found on our FAQ
<a href="www.beartoothbullets.com/faq/index.htm" target='_blank'>www.beartoothbullets.com/faq/index.htm</a>
keep us posted, and God Bless,
Marshall