There are several things you can do to reduce your group size.
First try using a black rectangle aim point stapled to a plain white background (I use multiple sheets of printer paper with the printed side facing the backboard). Make the black rectangle at least 6� �square.� It is best to have the rectangle taller than it is wide as you can then shoot for all four corners.
It is very easy to cant a laver action rifle when shooting from the bench. Using the rectangle you are able to align the cross hairs with the sides and top or bottom of the rectangle as you aim for the corners. This target will help considerably in eliminating cant.
If while aiming at the corners of the rectangle you put three or four bullets close together and string the others verticaly or horizontally bed your off hand into the front bag and hold the forend when shooting. With larger caliber lever action rifles this method of shooting from the bench works best for me
Since your rifle has made at least one very good group we can assume the crown is not the culprit. I assume you are using jacketed bullets so tight spots in the barrel at the sight dovetail, under the front sight and at the barrel markings are probably not the cause of problems.
I would remove the magazine tube and reinstall the forend and the rear barrel band with out the screw to shoot the rifle single shot.
One of the things you are looking for as you remove the forend is the alignment of the magazine tube with the barrel. You may find that the magazine tube must be forced upward in order to install the forend and barrel bands. This used to be a common problem and is one of the reasons Marlin 336�s of 1960�s and �70�s vintage are found with the bottom of the magazine tube reduced and a loose fit between the magazine tube and the receiver. If the magazine tube must be forced up against the barrel in order to assemble the rifle it will begin pulling against the barrel with the first shot.
By shooting the rifle single shot without the magazine tube and with the forend loose you will eliminate this variable and answer several questions with one five shot group.
If you are still unable to shoot a less than 2� group at 100-yards I would then look at the fit of the vertical locking bolt in the sliding bolt cut. You can smoke it with a candle or mark it with a magic marker and assemble it and work the lever a few times then examine the smoke to see how the locking bolt and the sliding bolt fit together. If the two do not make god contact the sliding bolt is in motion as the shot is fired.
There are other things which may cause your rifle to not group well but these are the easiest to look at and are quite common problems.
