Pick Up the Smart Gun | Mission 4: No Hope in Hadley's A: CM Guide

For starters, go to the weapons locker to get some ammo, because you've probably used up a lot of it in the last encounter with Xenos. Afterwards, approach the crate where the M56 Smart Gun is. It works on the same basis as the other unique weapons: it's not added to your inventory. Nevertheless, the smart gun will prove very useful in the coming battles, so take it with you without hesitation.