At 1 gram = 0.0352739619 ounces, you're using 3.39 ounces of cannon grade so it's not much less powder, just granulation. I am planning to vary powder amounts now that I've ruled out beeswax filling and wadding over the powder. I will agree that 3.5 would be a max for 200 to 300 yard range. These Minie balls are short range. I can cast some very heavy skirt ones if I ever get accurate.
Here are some of the recovered rounds that were less banged up. There are a few more that slid up the hill and are too messed up to stand up. The taller ones were dug out of dirt. The banged up ones skidded off hard clay into rock.

The rifling is well engraved and the skirts are in good shape. I agree that blowing the skirt off would not be fun. The fellow I bought the cannon from, Steve Cameron, said it took hours to clean a cannon when he did that.
Since I'm running low on powder I'll see about trying some cannon granulation. All the data I have to start is from Steve Cameron who shot many, many rounds through the twin of this. He always used 1f so I'm following this. He is oriented to long range and ultimately sold his off and switched to a Blakely since he felt that this was too small for Grayling.
The rounds did not whistle. I have electronic earmuffs that I usually keep turned up high to amplify the sound. The canister was cool.
So you have a 2.25? Who made it and what do you use for a mould? Some of the moulds that look like the air pellets rounds look like they would require a light load.
Steve