I don't see where it is mentioned; try "smoking" the case then inserting to the point where it hangs up, give a twist, then withdraw. You will definitely then see the offending interference spot.
Some of the old chambers for the straight cases were not cut in a linear taper, but rather what I'll call a reverse bell shape. (cannot offhand remember the actual term for it) The base area is a bit bigger for 1/2'-1" then a squeezes to a slightly smaller straight taper thereafter.

If this is the situation then your choices then are 1) a custom set of sizing dies, 2) run a "modern" spec reamer into the chamber to adapt it, or 3) cobble together some other dies to squeeze the cases enough to easily fit, then fireform.

- Mike