RE: Marlin Micro Groove
All it is is instead of having 4, 5, or 6 deep lands and grooves it has about 15 shallow lands and grooves. The advantage is supposed less copper fouling the disadvantage is that they are picky about shooting lead bullets. However there are numerous rifles with micro groove rifleing that will shoot anything that you put down the barrel.
Basically it is just another type of rifleing. Some think it is great and some hate it. My 444 has a micro groove barrel and it has so far shot every thing I have used just fine.