Thats the nature of slow twist rifling. It take a lot more powder to stabilize the bullet in a slow twist. Shallow rifling seals the bore easier than deep rifling, but requires at least a wet patch every few shots to clean the fowling out of the shallow grooves. My Carolina hog rifle has a Montana .50 cal. barrel 1in 72" twist, and is 38" long. It doesn't shoot very accurately with less than 80 grains of powder, and is most effective hunting deer, and hogs, with a 100 grains of powder.
Hungry Horse