There are a few tricks, besides the EQ roll-offs, that you can use to situate things in an outdoor environment.
Reverb plug-ins typically have two main components: early reflections and reverb. Most outdoor locations don't trap in the acoustic waves, so you're going to want to reduce the reverb side of the plug-in significantly (maybe enitrely). Use the early reflections side to create the space. Even in the woods, you'll get early reflections. So you want some of that audio coming back.
For those wide open spaces that don't really lend themselves to those techniques (i.e. wide open field), I like to use divergence with EQ automation. To use the gunshots/explosions idea that you're referencing, picture this...You see a gun fire on screen, maybe 40% to the right of center. The gun's attack, that initial bang, would be panned there. Almost immediately after the attack though, you start increasing the divergence, so that the tail of that gunshot spreads to other speakers. Speed and duration is up to you, and how it should work in the piece. Now, just automate the roll of frequencies of your EQ to shift for the duration of the tail. The automation of the roll of should be fairly smooth and consistent.
You can combine the early reflections and the divergence tricks for "enclosed" outdoor spaces (alleyways, canyons, etc.), it will just require a different pace of automation for your EQ roll-offs. You'll probably also want to allow a little more of the reverb side of your reverb plug-ins into the mix as well.