I have the breech plug and 80 grains has been ok, I was just checking to see the max in the huntsman
What year is your Huntsman? First two letters of your Serial #?
The 2002 & 2003 Huntsmans with 7/8" had a 100 gr VOLUME Max for powders. They were only available with 24" barrels.
In 2004 the Sidekick was introduced, and both Huntsman and Sidekick used the 7/8" breech plug. The powder max was increased to 120 grains that year, and 24" and 26" barrels were also available.
In 2005, both the Huntsman and Sidekick were changed to 5/8" breech plugs.
100 grains of Blackhorn 209 is plenty in a Huntsman/Sidekick with any bullet. I have some loads that will handle 110 grains accurately, but most are between 80 and 100 grains for best accuracy. I have shot several cases of BH209 through a Huntsman (24") and Sidekick (26"), and I prefer the 24" 2003 Huntsman with one of my custom breech plugs over my other 14 muzzleloaders. These include Whites, Knights, T/C, and CVA.
Here are the recommended loads for the 2002/2003 NEF Huntsmans. Disregard the .45 caliber information, they never went to production. These .45 caliber loads are not to be used with the 70's era H&R Huntsmans either.
Here are the recommended loads for the 2004 and later NEF and H&R Sidekicks. I have a 2004 24" barreled NEF Sidekick with 7/8" breech plug and 2008 26" barreled H&R Sidekick with a 5/8" breech plug, both rifles use the same manual and loads.
I do not have a 2004 or later Huntsman Manual, so hopefully either you do, or someone else that can shed light on which loads are recommended beyond that point.