Hello Jason,
I have converted several barrels for alot of people and I never ever recommend use of full bore bullets.
Always use a sabot with smokeless powder. The sabot is your pressure relief valve, if you load too much powder the sabot will blow out and let the excess pressure go out the muzzle. I've been shooting and building smokeless powder muzzleloaders for several years and I've blown quite a few sabots while experimenting with different loads, I've still got all ten fingers and both eyes, although the old eyes ain't what they used to be.
Get some harvester sabots, part #H4540B and use with Hornady .400", 200 grain XTP bullets. If your bore is tight enough you will get good results with H-4198, if it's not tight enough the powder will not ignite and the 209 primer will blow the load out the barrel a few feet.
If you experience this get some Accurate 5744 and you will never have any trouble with ignition. I always recommend the 5744 because I know it will be trouble free, you won't get as much velocity but the deer will never know the difference.
Stay safe and always use a sabot.
GOOD SHOOTING!
Slufoot