Steve, go with PSB and A2400 it will save you a lot of time and trouble. I'm no expert, but you are not going to be able to fill that groove diameter with just using larger bullets. You'll run into chambering issues. With rifling that is that deep, I have had success with two different approaches at this issue in my 1894 44-40. One is the above mentioned PSB, the other was Swiss black powder, SPL lube and 5-7 BHN lead bullets. Both work well for me. And the more information I gather and read the more I think the deep rifling is fairly common on the pre smokeless era rifles. Just another reason why hand-loading is a big benefit for matching the components that work best with each particular firearm. At one time I thought I'd be able to load two 44-40 round, one rifle and one pistol. Boy was I wrong. In hind sight if someone wanted simple and just one loading I'd go with 32-20 a Colt SAA and a Marlin 1894CL. both can handle the 10gr of A2400 and both like the 115gr. bullets. But simple is not as fun or educational. Happy loading!
Have a great Holiday weekend,