I tested today new to me Marlin 336 in 30/30. I tried two brands of ammo: American Eagle with 150 grain bullets and Remington with same weight bullets. American Eagle had noticeable higher recoil and point of impact was higher too. What was amazing to me was extreme accuracy difference with both brands. Remington produced three shot groups in 1/2" to 1" at 100 yards, American Eagle was big disappointment with groups in one foot range or bigger. Some bullets produced key holes.
Is this indication of bullets tumbling and what can cause this?

Hmm, keyholes are bad. I'd be surprised if a .30-30 couldn't stablize a 150gr. bullet.

Two things - maybe three.

1. Much too heavy (long) bullets loaded. Might pull one and see that it's really 150gr.

2. Way undersized bullets. Measure the diameter down near the base.

The third thing kinda relates to the first. If the bullet stability is marginal, and the muzzle velocity is really slow, then it might not stablize. Still, even at very slow velocities, this shouldn't happen with a .30-30 and 150gr. bullets.

Since the Remington ammo shot OK, I'd say it's an ammo problem not the gun. The heavier recoil of the American Eagle ammo isn't right. Pull one and sort this out before shooting any more of it.

In addition to Mike's comments, there's always the possibility of bad bases on the American Eagle bullets. The slightest angle or damage to the base will affect accuracy far more than damage to the point.

My bet though, will be slightly undersized bullets as compared to the Remington's.

Yah know if you're getting 1/2 to 1 inch groups with your 30-30 and plain ole remington ammo, sounds like you don't have anything to worry about. Thats some good shootin. Now about the other stuff, I don't know much, but 150 grain bullets should NOT keyhole.

Remington forever! It does sound like an error with the American eagle of some kind. Did you shoot it last? Sounds like something on the rifle is loose to me. Just thinking.

I shot Remington again after I shot poor groups with American Eagle to check if something went wrong with the gun or scope, but Remington shot excellent groups every time. I shot same combination early this month, but with Winchester rifle and did not notice bullets keyhole but accuracy was not there, which I contributed to open sights and my not so perfect eye sight. BTW I had keyholes one per 6-7 shots.
I pulled some bullets and dimensions seams to be O.K. I wonder if Micro Groves in Marlin, specially if barrel gets warm and bullets marginally to the correct size and slammed at higher velocity may not engage them so as to shooting smooth bore.
American Eagle the way I understand is generic brand of Federal and seams that they use second quality components in that brand.
I am ordering Remington bullets ( I heard lots of good comments about them) and will try my reloads.