The game automatically assigns you the gun with the best specs (IIRC, this doesn't apply to heavy weapons). You can then choose to go with it or swap it for another model of that type of gun.
I'm not really sure how they'd pick some of them. For instance, I've got a sniper riffle that can hold a lot of sinks and gets multiple shots off quickly but is weak and I've got a sniper riffle that holds 10 sinks and can only get one shot off but is very powerful. Which one is better would depend on how you play and the tactical situation.