When you sight it in, you will find that it groups much better if you rest the rifle on the bottom of the steel receiver, just in front of the lever. Don't rest it further out, such as on the forestock or the magazine tube. These are somewhat springy and can give you large groups.
Using this method, I have found that most of the .336s will group 2 inches or better at 100 yards, with 150 or 170, whichever it likes best. I found that the "Micro bore" barrels tended to fill up with fouling pretty fast, requiring a quick couple of passes with a brush and bore cleaner after every 10 shots or so, or the groups got bigger prettty quickly.
The Marlin 336s are very efficient and reliable. :-)
Mannyrock.