Wyo,
I would NOT build a rifle with a sassafras stock. In my experience, it might be acceptable for a real small bore, but the wood is not suitable for a large bore gun due to its tendency to split and break. It is a relatively weak, brittle wood but exhibits good initial stiffness. Consequently it is a good tone wood, the traditional choice for dulcimers, but as the stress increases it breaks rather than bends. I would be concerned about a 54 caliber rifle having a catastrophic break as a result of pulling the trigger. Most of the uses of sassafras are uses where there is minimal need for strength, low stress elements in furniture and small boxes. Some pieces of sassafras are pretty, generally a light to medium brown and it can be found with pleasing figure, but all the sassafras I have known is light, open pored, and brittle once dried.
If your time is valuable to you, choose a wood worthy of that time.
Best Regards,
JMC
John Cholin