I understand that some folks use Pyrodex because they cant get real black powder where they live. If this is the case for you, go with 777, you'll be much happier. Pyrodex pellets are nothing more than a failed attempt at re-inventing the wheel. They don't ignite nearly as well (and as a result, gun manufacturers have come up with more "reliable" ignition methods, again re-inventing the wheel) and Pyrodex is FAAAAR more corrosive.
If you use 777, remember to start off with a 10% less load by volume than used with real powder.
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If you wouldn't go to a restaurant and by a steak substitute, then why on earth would you use a black powder substitute in your gun?