Investarms does or did make a Hawkens rifle. It had a 28 inch barrel and a 1-48 twist. I have a manual for the investarms and the Lyman Trade Rifle. They both came with the Lyman Trade Rifle I purchased. Now remember two manuals for the same rifle. The Investarm manual gives much lower max powder charges then the Lyman manual.
I have shot 100 grains out of my Trade Rifle with little problem. Mine is a .54 caliber. And its pet load is 90 grains of 2f powder and a patched roundball. Very accurate and deadly.
In my .50 caliber Trade Rifle I was shooting 250 grain XTPs and 80 grains of powder and it was really accurate. I would be comfortable shooting out to 100 yards for deer with that load. 80 grains is a lot of power.