Hey Jr.
You may already know all of this, but:
1. When you sight in your Marlin 336, you have to put the receiver of the rifle directly on a soft rest. Don't put the forearm down on the rest. Just put the receiver right down on it (in front of the lever), and hold the forearm with your snugly with your left hand. This may cut your groups in half.
2. Almost all Marlin 336s I have seen like the 170 grain load better. In other words, they seem to shoot them more accurately than the 150 grain. I don't know why, but given that there is not an inch worth of difference in the trajectory out to 175 yards, try shooting the 170 grains.
3. When sighting in, only load and shoot one round at a time. Don't fill up your mag tube with 4 or 5 shots. With each shot, the weight of the front of your rifle will change, and shift the zero somewhat.
4. If your barrel has micro-groove rifling, then I have found that you have to clean them out with a quick swab of Hoppes every 20 shots or so, or those small grooves get filled with gunk, and accuracy suffers. The groups will open up to about 4 or 5 inches and stay there if the barrel gets dirty. You don't have to scrub it with a brush, just run a quick wet patch through, wait about a minute, and then run clean patches through.
Doing this, I have never seen a Marlin 336 that would not group at two inches or less with factory ammo (assuming no damage to the crown or rifling.)
Hope this helps.
Big Paulie