Winplumber:
The ONLY way to know what your bores measure is to measure them. And even if they are .690 there is absolutely NO reason for concern if the barrels are in otherwise good shape. Just shoot the gun with low pressure/light load cartridges and enjoy it. You will never know the difference.
And... as someone else said, opening up the bores is counter productive... perhaps even dangerous.
The "13" is simply bore diameter as measured at final proof. However, honing over the years may have changed this measurement.
Best, Kensal
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