So yet another crack of that ol' chestnut with an idea that may just bring it in line. Lets get into it because it is straight up that simple.
All random shot weapons are now just set number of shots (except for those that it makes sense for in orks because those xenos always have unpredictable results). So now those weapons can have a set number of shots however there is a special rule I want to add to those weapons.
"When targeting Characters, Vehicles or Monster; half the number of shots this weapon has rounding down"
This now means things like demolisher cannons can have something respectable for an explosive weapon like 6 shots which will leave infantry going for cover but against targets like tanks and other big stuff those things will get less hits but the stats of the weapon balance it out (basically a demolisher cannon being 24", heavy 6, Str10, AP3, Damage D6 + the new rule here makes it so it can once again be a good takes all weapon it once was).
This is a simple method with no special extra bar what amounts to what could be called the "Sniper" rule (weapons or units with "can target characters even if not the closest target"). This way we can remove the variance factor from random shot weapons which is their biggest issue in being actually good.