Yes, you can do both if you wish to get an idea of both guns and other systems.Can I do both? Or do I need to choose?
And, to do a roll.
I make a 3d6 roll and need to get lower than my number in that skill?
Yes, it is a 3d6, and you need to get equal to or less than the target number. Every bit less than the skill roll gives you more information, and don't forget you have the Overall Skill Level to give you a 15- on these.