This is actually not a bug, it was an intentional shotgun tweak. In gears 2 the shotgun was changed such that when NOT aiming (aka firing from the hip), the shot is oriented however the shotgun is pointing during the animation, NOT to the target reticule.
You will notice that as your character walks, the arms move and the gun points up and down slightly with the animation; when you fire without aiming it simply fires however the gun is pointing, straight out of the gun. So yes this often means VERY innaccurate shots. But its not a bug.
You need to aim your shots. This was a good change since before there was almost no advantage to taking the time to aim, the shotgun was almost a good from-the-hip as aimed.
In a nutshell, gears 1 artificially made the shotgun accurate when not aiming, so you would be shooting forward even if your gun was pointing down slightly during the animation. Gears 2 fixes this by forcing you to aim if you want to be effective.
I assume the same is for the boomshot.