Are you doing a lot of rapid fire shots to get these or are you doing a count down with the shooter to get in sequense with the trigger pull?
BTW, in that first shot, does that guy have a 35mm around his neck? It kind of looks like my Canon Rebel XS.
Sometimes I'll let off a blast of frames if they're doing a full auto "mag dump" or something. Sometimes I just try to get their rhythm and shoot when they do. If you see the flash, you missed it. I'd say its about 50/50 as far as results go. Mostly, I shoot guys that are shooting a butt ton of ammo. I also find that if I slow the shutter down to 1/100th or so, I get more flashes, but I don't like the way they look. I like the way the look at 1/1500 or more, but I get less of 'em.