Bore scopes

which one?
smallest probe i see is 8mm which is 31 cal
That depends on how much you are willing and able to spend, $55 to >$250.

https://www.amazon.com/s?k=teslong+borescope&crid=2FJW8WN8ZPOPC&sprefix=teslong,aps,171&ref=nb_sb_ss_ts-doa-p_1_7

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FYI you will find so many other uses for the Teslong as well. I started using to inspect inside brass web areas for signs of case separation in progress. The real prob is you will end up buying enormous amount of new bore cleaning products to clean what you think you see. I added another shelf over bench to store the crap, I mean, wonderful products that did nothing. IMO, stock up on quality brushes, patches and JB.
 
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