Bullets and Paper

I added a solid pound of weight to my rifle and the beast is finally tamed. No longer does it kick me, but instead gives me a push when I pull the trigger. But I'm starting to wonder about my ammunition. I shot Winchester 130 grain and Remington 115 grain and 150 grain, but the absolute best grouping I had was with Hornady 130 grain. Go figure, but the Remington 150 grains grouped at about 3". Hornady's grouped in at 1.5" (or I guess I could say 1.5 MOA). And the Winchesters grouped in at 8". Yikes.
Monday I'm going to repeat this but actually use sand bags to prop the rifle up and see if it makes any difference in accuracy among the ammo brands.