I've always wanted a marlin 336, so finally got around to ordering one, beautiful rifle and the wood is gorgeous.  Went out to sight it in today.  Had a new piece of plywood up, stapled me a orange paper plate and went to work.    It was awful seemed like it was shooting sky high and like the poi would move after each shot.  Went back and grabbed a fixture that sort of clamps the gun while you shoot, still didn't have much luck, best I can determine is it's shooting a foot or more high at  100 yards. Now my eyes aren't the best but I never have problems punching holes with my ar.  I'm not sure what's going on, I guess I'll try it again on another rainy day at 25 yards.  Going to let my shoulder rest a bit and hopefully read some advice.  The rear sight is at it's lowest setting.

Update:  It was the barrel band joining the wood and the barrel.  The band was so tight I had to beat it off with a hammer.  It took a lot of sanding and filing to get the wood down to where the band would easily slip on and off.  I refinished the sanded edge with some minwax polyurethane satin.  Took it out to the range and it shot great, no vertical stringing and the sight no longer had to be on the last notch.

What type/weight of ammo were you using? In my limited experience with 30-30, both my rifles ( Winchester and mossberg) are a bit ammo sensitive, or in other words there is a big difference between what they like and don't like.

I have had the best luck with Winchester 150 gn soft points, the worst withe Remington.

Buck horn sights? Are you using them correctly?

Try different ammo.

Have a friend shoot it.

Check the crown.

Check the barrel to stock fit.

I was shooting remington core lok 150 grain, but I also shot some 170 grain through it to see if it made a difference.  Yes, buck horn sights.  Using them correctly, I had to think on that.  I'm putting the front sight in the bottom, or narrow v of the buckhorn.  Is that incorrect.  Is there anything wrong if I try to remove the barrel band to see if that's shifting the poi.  I cleaned the barrel before use with hoppes 9 and a brush, followed by dry patches.  The barrel visually looks fine, crown looks fine.  

I can hit a paper plate with a ak, sks, fal, and a ar-15.  I did not have any luck with this rifle.  The trigger is heavy but I don't think that was it.  It's wheat planting season now and it will be a while before I can play with it again.  I'll try to get a friend to come along next time and see if I'm retarded, but seriously I had the rifle braced on a bench.  I keep thinking I should have seen some sort of pattern or bullets going next to each other.

Scope it

I would try some different ammo, and maybe a little more practice with that particular rifle.

Good luck.

It's a RemLin....Look for a JM made rifle and sell that one tp someone in New York.....

I don't know if this will change it's shooting but while looking at it, I noticed that whomever installed the magazine tube didn't have the screw at the end centered into the cutout on the barrel.  It was pushing on the barrel and just needed a little adjustment to get it into the cutout.

I'd bet the sights and your lack of familiarity with them is causing a lot of the group size problem.

Microscopic sight alignment and sight picture variations can cause big groups.  AR's and AK's have vastly superior sights to the buckhorns found on lever rifles.

I have a JM stamped 336 and even it HATES Core loks. It looks like I am patterning a 12 gauge when I use them. Mine happens to love anything federal or Winchester. I would start with changing the ammo and see where that gets you.

OP this is good advice, try winchester or federal ammo and see what you get.

Was working with my father-in-laws this weekend--decent groups at 100 yards with federal 170gr

I'll try to pick up some different ammo, hopefully that screw contacting the barrel was also a part of the problem.  If the accuracy doesn't improve, I'm just going to try and unload it at a gun show.  It is a beautiful rifle though, and I'm going to do my best to make it work for me.

Try loosening and re-tightening your barrel bands to "just snug" rather than torqued down.  It should help barrel harmonics some.   Agree with other posters, try different ammo. Maybe some LeveRevolution.

Went out to burn some more of the remington ammo.  Did quite a few shots at 25 yards.  Attached a bipod to the sling hook and fired away at my orange paper plate.  Horizontally no problem maybe had the holes move 1/2 inch left or right of center.  Vertically I had a string spreading out 5 inches.  Forward band I had semi loose.  Any tips for vertical movement.  Shots were fired standing, but with bipod supported by bench.

Same here, I have two of them. They like Winchester Super X 170gr.