Figures are from the Lyman Black Powder Handbook 1st ed.
peak pressure for 60 grains in a 45 caliber barrel
in Lead units of pressure
28 inch, 5,660 LUP
40 inch 9,140 LUP
43 inch 9,720 LUP
substantially more peak pressure in longer barrels. Even with such a moderate load, peak pressure increases between the 40 inch and 43 inch bores.
compare the figures for the same charge in a 54 caliber barrel
28 inch 4,860 LUP
34 inch 6,800 LUP
43 inch 7,000 LUP
the same trend is apparent.
I would have thought the Pressure in the 54 would have been higher because of the much heavier ball. I also never expected the pressures in a 45 to be so high with a moderate load like that.
Just thinking out loud, but it seems like heavy loads would be more dangerous in a longer barrel than a short barrel. Not that I would ever push that limit anyway.
An interesting thing is that pressures drop between 45 cal and 54 caliber. Considering the larger heavier ball, I would have thought the opposite.