I have never played metal gear solid, but the only drug i know which will 'calm the nerves' - or to reduce hand tremors from 'stress' is a beta blocker like propranolol. It slows the heart rate and reduces the tremors in the hands. That should help the guy aim better.

Of course this is in a game, and you should not be taking propranolol in a survival situation to 'calm the nerves'. It would blunt the 'fight or flight' response which you would want to be at 100%. All meds in the kit should be for your own use only. Unless you're a doctor.