a heavier gun is not inherently any more mechanically accurate just because of the weight.
heavier guns tend to be shot more accurately for a number of reasons, including:
- less recoil causes less flinching
- heavier guns are easier to steady in non-prone positions
- ect.
but if you take a remington 700 pencil barrel in .223, put it in a 6lb chassis and shoot a group......then pop the action out and put it in a 12lb chassis and shoot a group.....the group sizes will be exactly the same.
now that being said, a heavier gun is generally heavier because it uses a thicker action, and a thicker barrel, which will give you a stiffer, more rigid system...which will aid in accuracy, or more precisely, a more consistent shooting rifle.