Hey Great White Hunter,
I shoot the Marlin 25MN which is essentially the same rifle as yours; minus the fancy wood and sights. It has the exact same barrel. Mine is really accurate with the CCI Maxi Mags; both the HP's and solid points. I've never taken official measurements, but at 60 yards, the bullet holes usually touch. That's as good as it gets!
I too have had problems with Winchester Ammo. Try the CCI's. Hopefully you'll have the success that I've had. If the CCI's don't shoot well, keep searching for the right ammo. I promise you have a very accuarate rifle and it's just a matter of finding the right ammo.
Regards,
Texas Squirrel Hunter :gulp: