Now I see its for sale.
I think once you work out a load and get used to shooting it, you will find a grin factor from the Mashburn B. I know that I really enjoyed my K Hornet. Not every new fangled cartridge is the Bees knees, so to speak. Those older cartridges have a special appeal and as far as being too weak for Pd's, i don't know of any cartridge currently commercially loaded that is too weak to pop a fur balloon. I would think that shooting a Mashburn to 300 yards is as tough as shooting a 223 to 500. Use it within it's limits and the "minimalist" approach is a lot of fun.
I agree with the advice here given to anneal. Talk to Todd Kindler at the Woodchuck den about annealing and loading. Though he has wildcatted to smaller calibers, the concept is the same and annealing will help you make brass that doesn't get ruined. Just the brass you got is worth about $400. Be careful to make it last and enjoy that rifle. It will certainly shoot, and if it doesn't, don't give up, go back and try again.
Saubier.com may have some guys who can help you with the Bee and Mashburn data.