Only .002 difference between Starline 56-50 and max diameter for your rim seat doesn't sound right. Is it possible that your rim seat is out of round due to the peening of the chamber from the firing pin? If so, it could be preventing a "normal" size rim from seating, because you are having to turn the .50-70 rim down to the minor diameter of the eccentric rim seat.
That said, I have not been able to make the Starline brass work reliably with the 1860 extractor. Part of the issue is the undersized rim, and part of it is the undersized head. In my experience, the undersized head allows the case to tip slightly to the right on extraction such that the right edge of the rim is against the right receiver wall, and the resulting gap on the left is big enough for the extractor to slip past--"sometimes." Prudent expansion of the case as far down toward the head as possible might improve reliability, but I haven't gone this route yet. The Starline brass works better with the Lane-type extractor, but I haven't wrung the bugs out of it yet.
Anyway, I would check to see if the rim seat is out of round and fix that before I enlarged it beyond original spec.
--DJ