Analog: I think you need to measure the lands again. Most of those barrels are right on at .540". Use a .530 RB( measure them, too!)and a .015 to .018" patch, and you should get good accuracy. I don't know why your are shooting American Pioneer powder. Use Black powder. You can order it shipped to your door, and if you buy a case(25 lbs.), the cost will be about $12.00 per pound, shipping and haz mat fees included. Put together an order with your buddies. You can order smaller amounts, but the Haz Mat fee is $20.00, per shipment, regardless of what amount is shipped. Powder, Inc. and Graf& sons will sell you powder at discounted prices for as little as 5 lbs in your order. There is another powder suppliers up in Maine that also sells at good prices. Check the member Resources section at the top of the Index page here, and look for " Links ".
When anyone decides to use something other than Black powder, they are on their own. This is a traditional Muzzle loading site, and most of us shoot Black Powder because its a TRADITION! Most of the subs have to be used up quickly or they begin to deterriorate from the air they are exposed to after you open the cans. Black powder remains stable and useable for many years. I shot some of Dad's powder that he bought in the 1950s, and it shot just fine.
Its fairly common to find grooves that are cut .007 to .008" deep in the Hawken style rifles that were made by CVA, and Thompson, and others( .554-.540 = .014; divided by 2 = depth of rifling). There are a few commercially made rifles with rifling cut deeper, but I don't believe any of the current manufacturers are doing so. Custom barrel makers can, of course, cut your rifling as deep as you want.