i've used a 0.215 oblique view rigid borescope for decades (left over from my safe tech days and WAY more $ than the teslong endoscope) but was always sad that i couldn't inspect my .20 cal rifles with it. along comes the teslong with a measured 0.197 ferrule. YAY! the ease of use, documentation, picture quality and archiving, and price are all great selling points. have great photos of my 6mm's and 22 -250 carbon rings and was sure the .204 ruger was going to be even worse. then the cold water.... the 90* mirror would not fit in the bore. the mirror tube is indeed 0.197 like the cam ferrule, but the edge of the mirror makes max diameter 0.204. no fit.... now i gotta think for a minute or two. i wrote this up so i wouldn't do anything impulsive in the mean time.