Welcome to the forum.
Your .50 cal rifle shouldn't have a problem with the Red Stags.
The .50's have been used to take American Elk and the males can weigh 320 to 331 kg (705 to 730 lb).
As galamb mentioned, you should buy some extra bullets so you can give your gun a good testing at different powder loads.
Also, if you can find any, trying a different bullet is worth while.
Each gun is different and each will have one type of bullet that works better than the others.
Finding the one that works best is a worthwhile task.
These guns usually have two different loads which work well with the other loads giving poorer accuracy.
Real black powder is (IMO) always better than the synthetic powders.
It's ignition point is lower and it is much less likely to mis-fire.
It does leave fouling in the bore and after a few shots the fouling may make loading slugs very difficult.
For that reason, having a few damp cleaning patches and a cleaning jag for the ramrod is useful.
When wiping the bore with these do not pump them up and down the bore.
Run the damp patch down the bore. Let it sit there for a few seconds so the fouling above it has time to soften from the moisture. Then, remove it in one smooth pull.
Doing this will prevent the process from knocking fouling off of the bore and blowing it back into the flame channel.
When your actually hunting and you have loaded your rifle it is a good idea to remove the nipple (cone) and dribble a few grains of powder under it. Then, replace the nipple and your ready for the hunt.
This little bit of added powder will easily ignite when the cap fires and the flame it adds to the caps flame going down the flame channel will insure an almost instantaneous ignition.