As William implies, Marlin still has Micro Groove barrels, and most shoot extremely well, even with cast bullets. In fact, I don't think I've ever seen a bad shooting Marlin, though rumour has it they exist.
I have one. A 336RC built in 1957, given to me by a friend. Doesn't look like it's had more than a few hundred rounds through it, bore looks good, crown is fine, but after 5 or 6 shots it keyholes any 170 jacketed bullet I've tried. Clean it, and go through the same thing again. For those first few rounds, accuracy is fine - 1.5" to 2" @ 100 yds.
I thought the bore perhaps was loaded up with copper, but I've tried everything from JB compound to an Outers electrode system without curing the problem. 30-30 barrels are just about impossible to wear out through shooting, and I know that this rifle didn't see that kind of use.
First rifle I've ever encountered that had a mystery I couldn't solve. Frustrating.