I have the same rifle, new to me last spring.....54 cal. Pedersoli/Lyman GPR flinter. She is a shooter, but I am glad I dont lug it all day! First time out with mine, I also dry/balled it. I am able to trickle ...slowly, true...4f into the flash hole, poking in with the touch hole pin. Be sure to poke in more than what you think you need, or you may just poop it further down the barrel, and need to start over.
I have to admit...I wish it was lighter, perhaps splitting the difference with the previous Lyman GPR incarnation which I also have.
I am planning to use the flinter this fall for our ML season, and hopefully get out today but it looks to be rainy.