I don't know if you're talking about a specific brand, but Cabelas has hawken-style guns in 50 and 54.

If you are near a Cabelas, go look at them. Also check the returns, I found a decent one in Hammond, IN, marked down to a reasonable price though I would have offered less still.

T/C made the best 54 cal hawken if you ask me. I'd be pretty surprised if you can't find one used.

If you do get your rig assembled, consider something other that buckshot for squirrels. It will make a poor pattern and the squirrels you do hit may be quite damaged.