I don't believe that there's any outdoor airsoft walk-on fields in the GTA. Generally it's just wherever people schedule a game.
Some fields have closed down, or will be used less, and some people have acquired new land to play on. I can't really give a list of outdoor fields that will be popping up this season, but keep an eye in that subforum for events.
You can look back over some old posts in there, to see where the common fields are, though.
For CQB, a lot of people find that a sidearm is all you need. Because it really is all that you need. Rolling with a primary is just to add extra fun if you're into it.
If you're hesitant to try an indoor game because you only have your pistol, I strongly recommend you try gaming with JUST your pistol. It's a lot of fun.
UA will let you do a walk around of the field as well. You can bring your primary out on the field, and see how difficult it is to maneuver corners and stuff, compared to your secondary.