Yes, I understand your query - until you see the actual pressure generated back in those days - the mid 70's by the powders available back then. I guess I should mention that about all of the BP available nowadays, is much more potent than what was available back "in those days".

A perusal of the Black Powder Handbook put out by Lyman back in the early 70's will show this quite well.

They used GO (Gearheat Owen) black powder, not GOEX of the 80's and now as well as C&H black powder of the 60's/70's.

The GO was better at producing velocity than the C&H, used them. Up here, I used both of them as C&H was oft times easier to get than GO. At that time, C&H was akin to a poor grade of musket powder from the early 1880's period. It was slow, dirty and developed low speeds and pressure per charge.

In their 32" bl. .58 Lyman (Italian) Zouave .58, Lyman tested right to 180gr. 2F producing 1,625fps at 6,500LUP (lead units of pressure).
With GO powder, that same 180gr. charge, .562" ball of 260gr. and .020" patch delivered 1,737fps at 9,200LUP. They use lead crushers as the normal copper crushers gave erroneous & inconsistent readings, at such low pressures as generated by the black powder of that day.

In my 24" bl. Musketoon (2010), I started off with 75gr. gr. GOEX powder of (roughly)today, along with a .575" ball and .022" denim patch. That was tighter than tight, as the bore of that rifle was .574", but still doable with the steel rod and a good whack with my hand on the short starter. That load registered 1,308fps, yet back in 1970, Lyman had to use almost 110gr. of GO 2F to get the same speed. 1970 115?gr., vsa. 2010" 75gr. = same speed. That is a huge charge difference = 40gr., yet the same velocity. Swiss is even 'better' than today's GOEX. With velocity, you also get pressure, it is pretty much relative.

Back when testing these PH rifles, that 120gr. load of 2f I used in he 3-band, might have generated something like 1,450fps and only 6.500LUP.

Today, you can get that speed with a tightly patched round ball and about 100gr. 2F GOEX in a 32" bl.