Most modern barrels are capable of sub-M.O.A. accuracy. Period. Most shooters are incapable of eeking out the total potential of a modern muzzleloading barrel. The occasional bad barrel gets past the inspection teams, and fitted to an action. This is rare, but NOT UNHEARD OF.
Idaholewis laps every barrel he purchases, new & used, using the Lee Shavers Barrel Break-In method of lapping. This is a good place to start.
The second is the crown, making sure that it is smooth, but most importantly, EVEN ALL THE WAY AROUND THE MUZZLE.
Third, is the action's bedding. Is it solid? Is there a recoil lug? Is it loose? Does the action/recoil lug need to be glass bedded? Are the action bolts torqued to the correct amount of torque?
Fourth, is the barrel as it exits the forearm. Some barrels require solid contact for the entire length that the barrel contacts the forearm. Other barrels need to be free floating. Still other barrels need upwards pressure on the underside of the barrel at the tip of the forearm approximately the length of a playing card.
Fifth, is your breech plug. Is it correct for the type of powder you are trying to shoot? Does it leak? Does it need to be replaced? Is your ignition system hot enough for BH209, if that's the propellant you are using? Is your ignition system TOO HOT, especially if you are using traditional black powder?
Sixth, is your trigger. Is the length of pull correct for YOUR BODY? Is the length of pull CORRECT WHEN YOU WEAR YOUR HUNTING CLOTHES? Is the length of pull correct when you are wearing your COLDEST WEATHER HUNTING CLOTHES? Is the trigger pull TOO LIGHT? Is the trigger pull TOO HEAVY? Does the trigger pull have creep? Does the trigger pull have over travel? Does the trigger pull have roughness to the point where the trigger catches, stops momentarily, like a staged trigger? Does your firing pin spring need replacing? Does the firing pin itself need replacing? Are there any seals in the action that need replacing?
Seventh, do you habitually pick the flame channel/vent liner/flash hole/whatever name you want to call it, when shooting BH209?
I know that most people say that this is not necessary until the carbon starts to build up, but based on my experience with flintlocks, if you PICK EVERY SHOT, then the buildup NEVER HAS THE OPPORTUNITY TO CREATE A PROBLEM.
In 20 years of shooting flintlocks, I only had a couple of hangfires/misfires. They were early on at the beginning of my learning to shoot my first longrifle. Picking EVERY SINGLE TIME that I loaded the rifle, ensured that the gun fired 99.9999999999% of the time I pulled the trigger.
So, before you start obsessing about bullet, wad, & powder choices, make sure that you have eliminated as many variables as possible from the rifle & yourself as you can.
Don't drink stimulants like coffee, tea, energy drinks, cocoa before going to the range to shoot seriously. Don't get hyped up on tons of sugar, either.