Those are all great comments, Thanks... 410
I'll pull the gun out again and measure the inserts and try a 410 shell. From looking down the barrel. I'd guess the inserts are only the length of the shells themselves. That could be an optical illusion. So this time I'll do an actual measurement.
Whichever it was; looking at the clean bore tells me that nobody shot the gun. My Dad and Grandad were duck and goose hunters mostly; here in North Carolina. I live between Durham and Chapel Hill. If anybody knows a knowledgeable gunsmith anywhere close; please let me know. A machinist buddy of mine also suggested using a wooden dowel rod and a mallet.