Thanks, Steve:
I think you are onto something.
As documented in Dutcher's "BALLARD" book, and by Tony Beck, Arizona Trooper, there were even 56-56 rounds with larger OD bullets specifically for "Ballard" CW era rifles with oversized bores!
I recall Tony sharing that those larger rounds had a label indicating they were for use in Dwight, Chapin ... "Ballard's".
Blair has pointed out the odd dimensions in Marcot's chart, possibly from measuring at the crimp bulge. He and I have been discussing OAL to understand WHY there are differences in the lengths of lower block hooks in later models of CW and post CW era Spencers.
Subject to further information, it appears that the lengths of the "hook" on the front of the lower block range between .521" for early original 1860's and .660" for later ones, including Burnside Spencer's. That .140" is a lot. Two Burnside Spencer's and one Spencer rifle in the 23,000 range are reported by their owners to be nominally .640" - .660" give or take angles during measurement. I am waiting for a couple other measurements.
Blair and I are guessing that three factors are involved to enable easy cycling of varying OAL rounds:
The length of the "hook" in the lower block.
The distance the magazine tube follower extends the last round from the magazine tube.
The temper and top leaf profile of the cartridge guide "V" spring.
With correct OAL ctg's, swapping ctg feed guide springs can resolve cycling smoothness issues.
Perhaps rounds made later during the war were less uniform in OAL?
It is interesting to me that Larry Romano copied the hook and cam dimensions from a high number 1860, BUT tests with cycling dummy rounds that VARY in OAL between 1.400" and 1.600", to be sure variations will cycle. That's .200" OAL range. He's done this with me watching! Rounds with flat meplats with OAL'S at 1.57" cycle well in his. He did not say, but my guess is he includes some nearer 1.400" to be sure any with loose bullets that might have ended up shorter during cycling will work.
Not sure if you posted or emailed me that NOW your M1871 will cycle BOTH the 56-56 with normal OAL, AND the longer 50-45 ctg's, please? If so, maybe your longer hook is why.
Modified to include the 3rd factor, ctg feed guide spring. Sorry.
Thanks,
El Supremo/Kevin Tinny