marlin uses microgroove technology; multiple lands and grooves, usually about 12 or 16, I can't remember, but they are all very shallow. the thinking here is that, less bullet deformation and drag, as the bullet moves down the bbl., hence better accuracy.
Marlins , especially the 7000's and 2000's, even the 60 and the 795, and the magnificent 880sq, can typically run with some really expensive big boys out there, as far as accuracy goes.
the cb's are usually a conical ball; not made to the greatest of specs, proly not inspected all that well, also usually not checked for the diameter, which I believe should be a 5.6. also it is a 20 grain pill, a very short 20 grainer. So a lot of rifles out there, just can't stabilize these little suckers. So that is why I say that is a damn fine group, really, 40 yards, or whatever.