With the greatest of respect to 35 Whelen, this internet myth has been conclusively debunked. Marlin's micro-groove rifling handles APPROPRIATELY SIZED cast bullets at least as well as traditionally cut rifling. The problem, and smoke from whence this internet myth arises, is with folks using undersized cast bullets which, while a problem with any rifling, is even more so with the micro-groove rifling which is somewhat prone to more rapid lead fouling than traditional cut rifling. In other words, it is myth based on some substance, but myth nevertheless. Cast bullets sized to groove depth will work as well in micro-groove rifling as in traditional cut rifling. Undersize cast bullets will not work well in Marlin micro-grove barrels.
My problems with the Micro-Groove rifling and cast bullets occurred long before the advent of the internet. I have an early '70's vintage 1895 in which I tried cast to no avail. I still have the .459" sizing die I purchased (again, before anyone on the internet told me to) that I was sure would solve all my problems. Even the 49th Edition Lyman manual advises to use only hard alloy and warns the keep velocities 1600 fps or less in Micro-Groove barrels.
I didn't intend to come across as flatly stating that Micro-Groove barrels would NOT work with lead. My point, obviously not well conveyed at 1:40 in the morning, rather, was that unlike rifles with standard rifling, Micro-Groove barreled rifles would likely take more work to find an alloy/bullet size/load combination that would perform acceptable. Conversely, for my Rossi 92r I assembled a random 38 Special load of 2.6 grs. of Clays and a soft cast (9.5 BHN) 160 gr. SWC out of an old H&G 4-cavity mould I've had for years. First load, first try, 1 1/2" groups at 50 yds.
If the OP doesn't handload and specifically if he wants to shoot lead bullets but doesn't cast his own, the Marlin is liable to be a disappointment as he'll have little to no control over bullet diameter and alloy of which the bullet is cast.
I really wanted to buy a Marlin to use for CAS but had to choose between an older model which I new would be of better quality, yet have Micro-Groove rifling, and a newer model (post-Remington production)which I've read over and over are of questionable quality yet have Ballard-type rifling instead of Micro-Groove (Hmm...wonder why they changed the rifling?
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To the OP, if you'll shoot nothing but jacketed bullets, go with a Marlin if for no other reason, they're a much easier rifle on which to mount a scope.
Good luck!
35W