8 bore is 8 lead balls to the lb, so a 2 oz (ie 2oz x 8 balls = 16oz = 1lb) load should be spot on, even a tad light as a service load, given other gauges often enough have shot weights a little beyond their round ball capacity.
Armbrust's loads tend to be in the 1 1/2 oz - 2 1/2 oz range, both BP and nitro.
Somewhere in Greener is suggested a service load of 2 1/4oz / 6 drams.
The guns were made for it, and I just see no point in putting a tiny load through a big gun.