12 bore build almost finished

[Blackpowder8bore]

I am having a 12 bore percussion rifle built,28inch barrel,open sights,it's going to be very pretty. I know the max load I was given by the maker was 275grs powder and a 550gr 12 bore ball. I don't think I'll be needing that much 777 powder,but it's nice to know its that strong. I have a question.
If I'm using .715 round balls,should I use .015 patches or .010?

 

[Sparkitoff]

First , if you're gonna patch the ball you need to know the actual bore diameter. In a 12-g that could vary a bit from .720-.735 or thereabouts. Second, I'd consider scrapping the patch and using the biggest sized ball that will fall down the barrel on its own. Try both and figure which is better.

 

[SciAggie]

I shoot a 12 bore English Sporting rifle. I use a lubed felt wad over the powder and an .018” pillow ticking patch in my barrel for best accuracy.
I could not imagine touching off 275 grains. I hunt with 90-120 grains of Swiss 2F. 90 grains gives excellent accuracy for me. I keep my shots under 100 yds though.
You’ll enjoy the rifle.

 

[52Bore]

Best of luck..
I hunt with an original John Manton early percussion 17b (0.648” - 400gr RB). 60gr OE shoots well as I keep everything 50yd or under. Still exits thru double lung shots.

 

[BS]

What is your bore size and how deep are the grooves?