In most factory percussion rifles made today they can work but you have to load them with the built in 'starter charge' located at the rear, closest to the breech plug.
When I say most, I'm talking about the percussion guns with a powder chamber or "patent breech" design.
The percussion guns that have a side drum going into the side of the bore won't usually make the pellets ignite because the jet of flame will hit the side of the pellet rather than the "starter charge" at the back of the pellet.
Since you got them for free, go ahead and use them for plinking shots. Any shooting is better than none at all. :grin:
As for folks thinking about buying some of these pellets from any of the synthetic black powder makers I don't recommend it.
They are very expensive and using them limits the amount of powder you can load.
A lot of .50 caliber guns shoot well with a powder charge of 70 grains. A lot of .54's shoot very well with a powder charge of 80 grains.
If a person is using the 50 grain pellets, there's no easy way of getting either of these charges.