I have an older marlin 336 30/30. Shoots like crap and I've tried evey factory ammo made. Changed the scope out 3 times with nikon, leupold and zeiss. Still best groups I could get was 7.5" and even reloaded I finally got the rifle to group a .971" three shot group with the same factory ammo that I tried before and the same scope and base/ring setup as used before. So what did I do?

Please have fun with it and I'll let you know later!

Will it repeat that group?

I'd try 5 shot groups and see VAnimrod's question. It could just be a chance good group. They are a lot easier to get with 3 shots vs 5 and the 5 shots will tell a better tale when things aren't going as expected.

Other than size, what do the groups look like?

Tightened/removed barrel band?

Scrubbed the bore and removed fouling.

I have always wondered how tight the barrel band should be on these guns. My Marlin has one at the muzzle end, how do I tighten it to the proper tightness?

Got someone else to shoot it?

LOL ! Get someone else to shoot it! LOL . Dude! LOL. Sounds like it had a bad spot on the crown or some lead or copper fouling .
If its sure enough clean and the crown is good it will shoot .

I'm waiting to hear this one.

Did you program the bullets right?

Make a little bit of free play in the magazine tube?

Shorten magazine tube?

Shoot it single shot?

One piece firing pin?

Bruce

Just snug not tight works for me. And be sure the mag is not binding against the barrel.

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Was this ever answered? The answer probably is to move the bench from 100 yards to 10 yards.

Could be a fluke. Do 5 3 shot groups.

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.