"I shoot an old gun with the closest thing to the ammo it was made to shoot."

Amen, and not hard to do.

Standard loads found on L.C. Smith hang tags:
12 gauge - 3 dram 1 1/4 oz. shot @ 1165 fps (1887 - about 1920)
... 3 dram 1 1/8 oz. shot @ 1200 fps (after about 1920)
16 gauge - 2 1/2 dram 1 oz. shot @ 1165 fps (introduced 1896)
20 gauge - 2 1/4 dram 7/8 oz. shot @ 1155 fps (introduced 1907)

The pressures of those loads, esp. with Dense Smokeless, was very similar to today's loads
https://docs.google.com/document/d/1F2sQuPm05IE4VWYYnCkvuXmYEzQoWd_SQgaAfUOZEFU/preview

https://sites.google.com/a/damascusknowledge.com/www/home