Not sure where your location tas, is.

If I had an H&R, I'd first measure the groove diameter, then buy a round ball mould that size.

I'd purchase a Lyman shotshell loading handbook, and some Bluedot or other powder listed in it's sulg and ball loading data.

I'd purchase the wads they used and be happy with getting around 1,500fps to 1,600fps with up to a 600gr. ball.

If I wanted more power than that, I'd start at page 1 of Ed Huble's thread on 12 Guage From Hell thread and learn by reading the whole thread. You will find reference to the heavy barreled H&R along with suggestions for loads in 2 3/4" asnd 3" hulls along with 3 1/2" if your's is chambered that way. The loads will be for brass cases you need to buy, and reduced loads for use in the plastic cases.

If I didn't want to buy any books, or read Ed's thread and learn that way, I'd pull the lead shot loads from a fast magnum load, pull the wad up and cut off the shot-wad's petals and drop in a round ball of the correct size - and be happy with 1,300 to 1,350fps. That load is still as powerful as the lightest Africa 12 bore load of 1890, ie: the 4 1/4 dram black powder load- around 115gr. of black powder and will handle anything in North America.

You're in the driver's seat.