Taper in a shotguns chamber has been in the area of .0045" to .005" per inch virtually since the dawn of the breech loader. Likely what you have is some undersize chambers, possibly cut with a reamer which was sharpened a few times too many without replacement.

Especially in years past chamber dimension tolerances were somewhat loose.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra