A fresh charred barrel is going to be quite intense for beer, especially one that small. That doesn't mean you can't put beer into it, but you'd probably be looking at weeks (or less) before it's ready to back come out. And you'd be looking at wood character but not the aged character that comes with most "barrel aged" beers.
Depending on how long that barrel has been sitting before it ended up in your hands you may need to swell it up a bit as well so it doesn't leak. Standing the barrel on end and filling the exterior of the head with near boiling water, flipping, repeating, and repeating again a few times a day, and hammering the rings tight as you go, is my preferred way to do it. Filling the inside with near boiling water is faster and pasteurizes the barrel (to the extent that it can be...), but strips character and can create wonky staves if they swell unevenly. If it's fresh wood I'd be less concerned about sanitation (though less than if it held spirits, but moreso than if it held wine or beer)