In prone/crouch position. Adding a rocket launcher to your inventory, and afterwards picking up ammo for it. Will make you stand up, switch to the launcher, put ammo in it and then switch back to main weapon. Leaving you in a vulnerable position.
This is not a very safe thing to do in midst of combat situation.
Rather make it so that you could manually reload the launcher, in your own time.
This is also relevant to picking up a sidearm, and adding ammo. In this case he will first insert the cartridge, and then stand up and switch back to main weapon.