Whew! Couple days away, and a brushfire breaks out!
(That's actually good; this was a great topic to begin with and the drift has been just as good!)
Mako, my curiosity about the 44 Colt chamber is because my 1872 Open Top is chambered that way, and I'd want one type of ammo to fit all guns. Your bullet design, seated out, looks like it'll eat up most of the Specials chamber which made me a little concerned it might be too long for the Colt?
While I think your design is ingenious, I'm gonna keep pondering this dilemma. With the brass being longer, but thicker with solid-head 44 Russian brass, there is really no perfect solution I can see (yet). My biggest stumbling block with the designs you posted is that I'd have to grind away so much of the little flapper on the Adirondack lifter, that I'd worry about reliability and resiliency. What do you think?
1.345" does seem to be the right length, regardless of case length, bullet weight, or powder charge. (Which makes sense, since new ammunition designs still have to work in the older rifles and with no cartridge stop, that demands fixed-length ammo...)
I'm guessing the "44 Henry RF Flat" was an attempt to give the Henry/66 designs more power (like superformance ammo today). With the cartridge length restrictions and heel bullet design, the best way to improve the Henry was to add bullet weight. A RNFP puts more lead outside the case than a same-weight & diameter bullet of conical shape (like the original Henry), so that's most-likely my guess of where the 216gr bullet came from.
My next supposition is that Winchester probably started off with a 220 or 225 gr bullet (10-12% increase), but then someone noticed velocity fell off enough that striking energy didn't improve significantly. By judiciously juggling case length, bullet shape and weight,, 2 more grains of powder could be added to retain the same velocity, but now with an 8% heavier bullet, which was still a good improvement for those days.
Lots of assuming on my part, but it easily explains the combination of changes that happened.
I wonder what date the Henry
Flat cartridge (as opposed to the original conical-shaped Henry cartridge) came out? If it came out around1866-1868, it could also be an example of marketing (make the new model 1866 more interesting to customers, because it has "more powerful, new loading of the Henry cartridge" type thing). Remember that this is around the time lots of Henrys, Spencers, and a multitude of other guns were hitting the shelves cheap due to the end of the war. Winchester was probably pulling out all the stops.