I am assuming this is a 50 cal rifle, right??
If so, I would recommend you not try to push your powder loads into the "magnum" range, i.e. over 120gr.
My older Knight USAK, which still uses percussion caps and has a barrel 2" shorter than yours, likes medium powder loads with heavier bullets. I have been using 777 powder (which is probably a bit hotter than straight Goex) so you might have to tweak my loads to get the best accuracy for your rifle - but here are my general loading specs:
300gr sabot with 80gr 777
350gr sabot with 90gr 777
400gr sabot with 100gr 777
All of these loads run approximately 1500fps out of a 22" barrel and render tremendous accuracy and killing power out to 100 yds. Here is a post to some targets I've shot with them:
http://home-and-garden.webshots.com/album/66414501RJKYAr
The Precision Rifle QT bullets are my 1st go to bullets for hunting - their performance has been impeccable. The 300gr Hornady XTP Mags are great for sighting, paper-punching, and maybe hunting too - I just haven't used them on a hunt yet. The 300hr Hornady SSTs also provide marvelous accuracy.
I really don't think your choice of 209 primers will matter too much since you are using true black powder instead of the substitutes. Most of the new 209s made for the substitutes were designed to lower the incidence of the dreaded "Crud Ring" that 777 is famous for with 209s - this is no issue for Goex Black.
Hopefully this info will get you going for now - you are gonna LUV muzzleloading, it is SO addictive.