I have a Leatherwood Malcolm 6x scope on a green mountain 50cal barrel. I wanted to know how good these scopes are? Are they durable? Are the mounts that come with it any good? Will this hold up to recoil of heavy 50 cal loads?

You are going to have to buy mounts.  I tried one on my 45-70, and I may go back to one  eventually, but wanted a hunting rig and while the vintage look appealed, the performance was more important.

The scope does not come with a Pope rail to allow it to slide (or it didn't when I purchased mine).  If you're not shooting a low wall in a caliber that starts with a "2" you are going to need one.

The mounts from Leatherwood, at least the ones that come with the scope come with soft steel slot head screws, you'll never get them to torque w/o buggering or stripping. 

The front mount for an 1885 was supposed to fit in the dovetail for the rear barrel mount sight, it was an awful fit, I'm talking see light under it kind of fit. 

The quality of the scope itself seemed fine, but without the right mounts it's not going to work.   I may eventually go back to the scope and get a set of mounts from MVA.