I think they wanted to try and avoid using guns as much as possible because they are loud and can attract unwanted attention. Once that fence fell over, not much they could do except shoot. Probably the last thing they would want to do is to use guns.

definitely it is the last resort if all else fails because it will attract another wave of walkers, it was explained in the comics and i guess it will explain it on the show as well. I hope they attract those thousands of walkers on the highway to get things more interesting lol but i was thinking during season 3 they have those silencers on their guns why didnt they use it on that time...i guess they were panicking already.