The sage of the sOuth is correct that it is impossible to know the wall thickness, or evidence of active corrosion, at the bottom of pits. The wall thickness gauge pin can not reach the bottom, and obviously the medial barrel surface of a double gun barrel can not be evaluated.

We do however have excellent tools to see every mm of our barrels.
The Hawkeye® Pro Hardy Direct Borescope would be the $1500 or so standard.
http://www.gradientlens.com/Rigid-Borescopes/Hawkeye-Pro-Hardy.aspx#6861-overview

Lyman sells a rigid digital scope
https://www.amazon.com/Lyman-Products-Borecam-Digital-Borescope/dp/B00URUE84W

And there are flexible digital scopes for $13 to $130

This one is $38 and highly rated
https://www.amazon.com/Endoscope-Depstec...R1VCH5FEAX7YM71

Really no gOOd excuse for not knowing the condition of our vintage barrel bores

re: cleaning I would avoid the stainless steel Tornado brush; they are available in brass
https://www.amazon.com/Pro-Shot-12-Gauge...BRN5HZE89BMD9JR

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