One choice you have is software that generates muzzle flashes. Fxhome sells a product called EffectsLab that generates muzzle flashes, creates particle effects, and makes "light swords" (lightsabers). The interface is very easy to use and the website has extensive tutorials on this product.
Here is a basic muzzle flash. The 3D controls make orienting your effect very quick.
Here are just a few few of the controls. Making custom effects is really easy, plus you can save them for later. Over all this is really good software and they keep improving it; but I really hope they would develop it into After Effects plug in. Check out their site for tons of user videos.
Your second choice is stock footage. I HIGHLY suggest using a lot from this category. I think a modest muzzle flash combine with these are a slick combo! Why? It gives you a great spark and the much needed smoke. If you watch real guns shoot it is almost all smoke.
Still from NoControl Cinema Clips
Make a trip to these places and see their selection. They both sell HD muzzle flashes, this is the bread of your war film (sound is your butter).
Your third choice is photoshopping some flashes. There a few tutorials already made I'm trying to get my hands on. Soon as that happens i will post them here.
Fourth choice is The Foundry's plug in Tinder Box 4, it works in After Effects but just doesn't look at good as Effects Lab or has enough controls to make the plug in work it. The color dots are a watermark because i was trying the demo. Other plugins from this serious look great, this one just isn't developed enough.
Fifth- Delirium for After Effects: I didn't even bother loading the demo, this software is old. The main reason i didn't even give it a change is the manual states it is only for side shots. End of story.