More is not always better

[8X56MS]

I am continuing to work on my 30-30 loads. My little 336Y Marlin with the 16 inch barrel is still happy with the factory Remington 150 grain Core-Lokt ammo. These group between 1.5 and 2 inches at 100 yards.
I wanted to try the load I worked up for the Savage Model 170 pump, in the short barreled lever gun.
I first tried the original load, which was 34.5 grains of IMR4320 pushing a Speer 170 grain flat point jacketed bullet. This gave a group much like the 150 grain Remington factory load.
Then I tried the second loading, which is 34 grains of the same powder, with the same bullets.
Group size dropped down to between 1 and 1.5 inches. Going back to the 34.5 grain load, the group size went back to the bigger spread.
I tried switching several times, with consistant results. The load with 34 grains of 4320 is just more accurate in the Marlin.

In the Savage 170, the difference could not be seen, but with the lever gun, it was dramatic.