Sorry that I didn't see this earlier. Here is a thread related to muzzle loaders that may help. It contains a lot of info. Be careful not to over-charge those older rifles. New Muzzle loader and questions (newbie)
Here are a couple T/C Links that may help. If you search the web., CVA may have a similar link to manuals. Charts - Thompson/Center
It states that your 54 cal. Hawkins has a 1:48 RH Twist. Muzzleloaders - Thompson/Center
Go to the left side of the Home Page - bottom of the list & click on Manuals to find your Hawkins.
It's been my experience that the 1:66 & 1:48 rifles are best with Patch & Ball.
A 1:38 twist will shoot either as long as the Sabot Bullet weight is 200 gr. or maybe 230 gr. The heavier bullets topically don't stabilize very well. The newer rifles with a 1:28 twist work best with the heavier Sabots.
Check out the Thread listed above and if you have more questions, I'll try to help.