Four drams and 771 grain bullet would be a banging load for sure.
I doubt that's the regulation load, but you could try...

My 16bore light rifles have done well with 2-3/4 to 3-1/4 drams powder and 1 ounce bullets. I have one that runs more powder, but it's unusual and a total ripper.
I'd guess a start in the 3 dram range with Fosbery paradox-style bullets cast 20:1 and sized to the diameter of the cylinder portion of your bores would be safe.

I stirred the pot with Cheddite years ago for a short production run of 16bore paper cases. I don't think they've made them since. Plastic will work fine.