Sherwood,
I have seen several Marlins that shoot like yours or worse. The cause is almost always caused by the front sight installation. The screw holes are sometimes drilled too deep or the screws excessively tightened resulting in a slight bulge inside the barrel.
When the barrel is clean you can see it with a good light or a bore scope. A tight patch pushed through the bore will let you "feel" it also.
The only way to fix it is to cut the barrel off behind the front sight, re crown and CAREFULLY re install the sight..
Good luck,
Lefty
