Is it a 24" or 26" barrel? The 26" Sidekick is listed as a MAGNUM (150 grains pellets), my 2003 24" barreled Huntsman is listed as 100 grain maximum (loose powder or pellets). Did you get an owners manual with the Huntsman barrel?
All the new Huntsman/Sidekick barrels are 26". The Sidekick says 150 grain loads are ok, but I have never heard if they list the 26" barreled Huntsman as being capable or not. The barrels themselves are dimensionally identical, it's the lug and receiver area that is different.
This is what it says for my 2003 Pre-Magnum 24" barreled Huntsman. Disregard the .45 caliber stuff on top, they never released those. Note that these are just recommended loads for best accuracy in the 24" barrel.