That's too far for what you are shooting, if you are trying to hit a conventional target OR hunting. In fact there are no commercially available barrels today that will work.
What you're trying to do has been accomplished before. As a reference you need to refer to
The Sporting Rifle and Its Projectiles by James Forsyth. Forsyth tried and succeeded in doing exactly what you appear to be working on.
The problem for a hunting rifle is two fold. Accuracy for one, and enough velocity and inertia upon impact to take the animal. To launch a patched round ball that far with a flat enough trajectory, and enough speed at impact you will need a larger charge than you so far are using, and probably a large caliber.
You will need a custom rifled barrel of 1:104 twist rate or slower, perhaps as slow as 1:120, AND custom rifling as well. This allows the patch to survive the explosion of the powder that is needed, and as a bonus will give a very low amount of fouling. As for the rifling, you need very thin and sharp lands to help hold the patch at the high velocities that you are generating. Foryth recommended that such a rifle be .69 caliber.
(Please excuse the illustration as the Forsyth should have 6 lands, not four as illustrated. The illustration is to provide an idea of how different the lands are from factory rifling)
LD
