If you hold down the right mouse button with your sights up (any sights), you hold your breath and become steady for several seconds. Try that and see if it helps.

Centipede is right in that many under-barrel weapons (Master-key, m203) Are not easily removed from certain weapons. But there are some that are designed to easily be replaced on the field using a standard rail mount. The HK 416 is an example of this. I cannot remember the name of the launcher that is paired with it but i'd imagine that in 20 years, most if not all weapons would have easily detachable under-barrel mounted weapons.
And as other's have commented, this would open up the field even more for forward grips even maybe second rail attatchments (IR laser and flashlight)?