I've got a stock Marlin 336C with a Leupold 2.5x LER scope on the lever scout base. With factory Remington and Winchester loads, I was getting 3-4" groups at 100 yds. I too thought this was a little weak.
About this time I started handloading. The first load I tried was out of the Speer 13 manual, with once-fired brass and the 170gr Speer flat point. Five shots into 7/8"! I have been able to repeat this on several occasions. Oddly, even with the maximum Speer recommended load of IMR4895, velocity is a couple hundred fps lower than the factory stuff. I have been told the Speer manual is too conservative, but the accuracy went down as the velocity went up, so I am happy with the load as it is.
I have done some experimenting since then, and found that my gun definitely prefers the heavier bullets. Groups open up to 1.25" with the 150gr, 2+" with 130gr, 3-4" with the 110gr, and we don't even need to talk about the 100gr Plinkers. I talked to a Speer representative at the NRA show, and he recommended even higher speeds for the lighter bullets, but I won't go there until I see hotter loads published by reputable sources.
Any chance you know someone who can put together some handloads for you to try?