I'll try to answer your query as best I can, John

As to the heavy bullet loads in the .450 BPE (or maybe it was factory BP 480gr. loads as well in a .577/450?) - John Taylor wrote of using them at one time, as well his Martini when his other rifles went swimming in a boating mishap.

I'd sure he did mention using heavier bullets in the .450BPE because the factory type 270 to 300gr. were not suitable for dangerous game. IIRC

As to the Sharps ctgs. and buffalo, one needs only read Frank Seller's book on the Sharps, along with Firearms of the American West, 1866 - 1894. Yes - there were many .45/70's used as that was the military round and ammo was available at any Fort on the frontier. Too - many more .50/70's were used by civilians after the .45/70 ctg. became the Army round.

There is some verification that the "Big .50" that Billy Dixon shot the Indian Chief with, at 1,535 yards, was actually a .50/70. Many books say it was the .50 2 1/2", however THAT round was not quite a year old at the time that Dixon made the shot. The jury is out on what exactly was used. Dixon said himself that it was a 'scratch'(lucky) shot.

Elmer Keith once said that hitting & killing at long range was accidental, but if you practiced shooting at long range, you'd have more long range accidents than someone who didn't. Of course, Elmer was talking about shooting handguns, but the same facts remain true to rifles as well.

The military Sharps and Rolling blocks were sold quite cheaply to 'settlers'. Factory Sharps and Rolling Blocks along with Ballard single shots were also popular in both .50/70 and .45/70. This is all common knowledge from History books as well as from Sellers book. Both books mentioned have copies of correspondence from the more literate hunters themselves to Sharps blowing eloquently their praises and of some shots made. Multiple letters spoke of the 370gr. .40 cal. as well as heavier bullet'd .45's killing more than one buffalo with one shot even at 300 to 400yards.

A 'number' of photographs from "The Day" show a single Sharps, sometimes a carbine, sometimes a longer barreled full stock both military guns, laying against the ribs of a dead buff or elk with numerous others dead on the ground within 50 or 60 yards, captioned mentioning how deadly these rifles were - were they dragged there with horses then all traces of that evidence removed before the photo was taken - doubt that. They rifles used were 50/70's as the .45/70 military rifles of the day were all Springfield's or Rolling Blocks.

At that time, the .45/70 load was only the 405gr. round nosed bullet with 55gr. or 70gr. rifle powder while the .50 ca. shot a rather pointy 450gr. both loaded with 70gr. There was a lighter 55gr. load for the .45/70 Springfield carbines due to complaints of too heavy recoil of the 405gr. and 70gr. in those lighter rifles.

This is intersting, as the .69 cal. muskets of 30years previous (up to 1955) used 135gr. rifle powder and a 400gr.round ball. Yeah- those soldiers complained as well. From the rEv. war until 1820, the musket load was 165gr. HA!

One of Gen. George Custer's favourite 'sporting' rifles for antelope, deer, elk and buffalo, was a .50/70 Springfield. Custer used to "wax-eloquent" about his shooting exploits, but a certain George Bird Grinnell who accompanied him on the 1874 Black Hills expedition, said "Custer did no shooting that was notable".

Yes - the .50's were popular but due to the prolific numbers of the .50/70 and .45/70's due to availability of ammo, the .50/70 probably reined as chief buffalo killer with the .45/70 being in second place. Many of the high money makers on the Plains, like Frank Meyer, used the long case .40's and .45's- ie: 2 3/4" and 2 7/8". Some even had Fruend (St. Louis gunmaker) re-chamber for the longer 3 1/4" case in both .50 and .45 calibres, however Sharps never chambered ANY rifle for a 3 1/4" case.

.45/70 issue loads - 70gr. rifle powder 405gr. RN- 1,320fps.- after 1800, increased bullet to 500gr., and a speed of from 1,250fps to 1,310fps.
.50-70 issue load - 450gr. 70g.r rifle powder, 1,240fps, I think it was.

My loads and those WE used in the .45/70 Siamese Mauser were:
68.0gr. H335 - 350, 400 and 500gr.
66.0gr. IMR4320 - 350, 400 and 500gr.

The H335 load used to make a ball of fire about 2' in diameter and when fired into the air at night, made a quite a display. closing ones eyes kept one from firing momentarily blinded.

The velocities were as noted - except I rounded the 350gr. speed from 2,290fps to 2,300fps. My rifle, with a 15" twist did not like 300gr. bullets driven above about 2,300fps, for some odd reason. I got keyholes when I drove them at 2,400fps using H4198 - yet 2,532fps is listed at only 46,700cup from a modern rifle barrel.

Note both these ctgs. allowed seating the bullet out, they were not restricted to 2.55" OAL, thus in comparrison, the chambers, useable powder capacity would make them the same as a 2.3" case.

A Hodgdon Annual Mannual (yes- a new one ever year) will show data for the low pressure guns, lever guns and modern guns, however their data only goes as high as 50,000psi in what they list as modern rifles. There is no reason I can see, for not exceeding that data - IN MY OWN RIFLES - as the .458 and other rounds with strong brass in that calibre are loaded higher - like the .458 mag.- to 57,000psi.

I am looking at modern .45/70 rifle data right now in Hodgdon's current book of 250gr. TSXgr. at 2,607fps- at only 44,607cup, 300gr. TTSX 2,445fps at 46,700CUP, 350gr. Hornady FTX at 2,378fps at 46,500CUP, 350gr. RNHorn at 2,300fps at 50,000CUP and 400gr. at 2,108fps at 49,100CUP.
They did not list the OPAL for these loads, but I assume they probably kept them to the same as the Marlin's 2.55". when you seat the bullets out, in comparison, you are effectively lengthening the case.

The lever gun Marlins in .458 Marlin and .45/70 are listed to 2,000fps with 400gr. in this manual, and 350gr. to 2,191fps to be exact- at 39,300 CUP.

I already know that a a Marlin .45/70 with an 18 1/2" bl. can run 2,004fps with a 400gr.

At safe pressures for the Baikal- listed as max at 32,000psi- which is also 32,000CUP, the numbers being the same at that level - apparently.

250gr. TSX - 2,118fps - 26,200psi
300gr. Sie. - 2,221fps - 27,600cup
325gr. FTX -- 1,958fps - 26,300cup
350gr. Horn - 2,022fps - 31,400cup
400gr. Spr. - 1,856fps - 29,600cup
400gr. Spr. - 1,915fps - 31,800cup - diff. powder.

I would be inclined to try Speer's loading data of RE#7 with the 350gr. bullets - from start to maximum. They state they held pressures to 28,000cup.

Note that the new 350gr. Speer bullet has a 1/16" (.0625") thick jacket and is designed for deep penetration in the .458 Mag. The single shots and Baikal can run that bullet seated at it's cannelure, which is too low for a lever gun (except in shortened 2" cases). They say it needs 1,900fps MV to expand - sounds like a marvelous big game bullet to me, for a first round .45/70 lever(in shortened cases) and modern rifles.

Due to the small nose size, I do not think I would stack them in the magazine, though, one in the chamber, one only in the mag. ie: 2 shots not stacked in the mag.