Rwalter63, Welcome!
If the book says only pure lead, I would assume that it is referring to patch & ball. When using this type of ammo, one is advised normally to use soft, pure lead balls only. Should not matter if projectile is housed inside a plastic sabot though.
I shoot a Huntsman which basicly is same gun as Sidekick. My Huntsman only comes with 24"barrel though.
I shoot everything available for a 50cal muzzleloader. No problems as long as you stay within the recommended charge size. There are as many different projectiles out there as there are flavors of ice cream. :grin: Really your choice and budget available for your shooting needs.
Once I load the gun (usually use loose powder rather than pellets) I don't like to leave the load in gun more than a week(during hunting season),provided it hasn't been rainy/wet weather. If so, I pull the breech plug...dump load,..reclean and reload. Just as easy to discharge gun in the woods,but I never seem to want to disturb my hunt area by doing so at end of day. :roll: Just me though.
If you have a misfire,do what you feel safest at doing. I usually wait a few minutes (obviously keeping gun pointed downrange), then use a nipple pick to help insure flash hole is clear, and try another primer. If that fails after a few repeats, I let gun sit about five minutes (always pointed in safe direction) and carefully pull the breech plug to dump the charge.
You can use a can of WD-40, with spray tube, and spray a few times into the flash hole of breech plug, to wet powder before pulling plug... if that suits your needs for safety sake.
To be honest, I cannot remember having a misfire with my huntsman,but have had a few with other inline brands.
Beauty of the Huntsman/Sidekick,is that they are so user friendly...and pulling the breech plug/cleaning is just SO EASY! :grin:
Back to projectiles: I personally buy most of my bullets (usually made by Hornady or packaged through T/C ) in boxes of 50-100,and buy my sabots loose in packages of 50. Cheaper that way for me and not running to store every week for ammo. Try some Hornady 240gr XTP's (.44 & .45 cal), T/C 250gr PTX's,or some 240 gr T/C Cheap Shots...as well as the 295gr Power Belts that you have,as Bubba recommends.
I would recommend starting around 75-80 grains of powder and working your way up 5gr at a time to see what you and gun likes best,with your projectile choice. Good luck!