That adds up as it would be very light for a 3 inch chamber

My early Williams & Powell DR in .577 Snider is 8lb 7 oz.

My .577 3" is 12 lb

1 in 96" is very slow twist

I am sure you will start off with light loads as recoil could be "stout`"!

I hope you don't mind me mentioning that Black Powder is what it was designed for

Should you use Nitro for Black magnum primers and filler rod/packing for such a large case

Good shooting!

TH