This is kind of a neat write up on the Rifle
The Navy Arms "Hawken Hunter" was a pet project of Mr. Val Forgett, the late founder of the company. It was designed to use Minie type bullets with huge powder charges, and was personally used by Mr. Forgett to take elephant, cape buffalo, hippo and other large game in Africa. The power charge used was 175 gr. 3FG, with a 610 gr. minie bullet. 58 cal. 26" barrel twist=1/60" Case Colored Lock with brass hardware. Designed to handle the heaviest loads safely and accurately. The Lyman Muzzle Loader hand book lists loads up to 190 grs of 2FF with the 577611 Lyman bullet @ 530 grs using Crisco for lube. Lyman book describes Val Forgett, owner of Navy Arms as shooting 180gr of 3FFF behind the 610 gr minnie (Lyman 577611) They got consistent 4.5" groups at 100 yards. See page 20 in this book for the entire six page story. They got elephants, hippos, wildebeest, sable, cape buffalo, lions, antelope, zebra, wildebeast, hartebeeste, bushbuck, duiker, impalla and others with the 58 cal big rifle.