The Plainsman Challenger Arms Corp.

Might help if you post pics, Plainsman Challenger looks to be a pump air rifle not Co2 unless there's 2 models with that name.
Is it a pistol?
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Might get a big wide bladed screw driver as thick as slot put it in a padded vice and give that a try, or put in padded vice put coin in vice grips and give it a try.. trying it while holding in your hand theres little leverage and your fighting grip strength.
If you cant get cap off then gun is pretty much worth whatever parts can be salvaged off it.

Heres a idea on value, keep in mind Mikes always a little high on asking price..