Micro-Groove barrels will handle hardcast slugs fine, but they must be properly fitted to the bore. If you want to shoot hardcast, slug your bore first and firelap if necessary. Marlin barrels tend to have tight spots under the rear sight dovetail and front sight screw holes. Firelapping is the quickest way to the bore one diameter. After that fit the bullet diameter to about .001-2" oversize of the groove diameter. Should work fine.

I'd try to find some hardcast around 21 BHN on the hardness scale.

Of course, this advice might be worth exactly what you have paid for it.

For more on shooting hardcast bullets in Marlins, especially the larger bore rifles, check out:
http://www.shootersforum.com/