And of course I understand that I am totally in the realm of conjecture here but I'm just putting forth a scenario which I think is consistent with teenage mindset, consistent with Dunn's account, and would explain why they didn't return fire, gun wasn't found, etc.
Explaining what might've happened after isn't too hard. Apparently the driver (the one with the felonies) came out of the store at some point during this argument and had gotten behind the wheel, hence their ability to drive off when Dunn started firing...
So, Dunn drives off and these guys are assessing what the hell has just happened. OMG Jordan is hit! He's fucking bleeding to death back here! What should we do? Drive to the hospital? Call 911? Did the store already call 911? What the hell happened? I went inside for two minutes and you guys get in a gunfight?
"This guy asked us to turn down the music and Jordan got upset and was going off on the guy, then he just grabs your shotty and points it at him to scare this fool"
Imagine what you'd think once you realized that your gun, which you illegally have, is in the car, the cops are on the way, and if the driver knew enough about the law he might've also known he could actually be on the hook for Jordan's death. Regardless of knowing that though, he'd know there was deep shit heading his way if the cops show up and find that gun.
So the remaining 3 are talking a mile a minute at this point and their stress levels are absolute maximum. The driver knows the gun has to be gotten rid of and they don't have a lot of time. They've already driven part of the way over to this nearby parking lot, and he sees a trash can or bushes or something. They get out to check on Jordan, etc, and the driver ditches the gun without drawing too much attention to doing so. Then they drive back to where they'd been parked.
If this or anything like this happened, they've got 4 days for that gun to disappear one way or another from that spot. Tons of ways it could happen. Someone finds it and takes it, it's in the trash and the trash gets picked up, they return there later that night after they've been released from questioning at the station and retrieve it themselves. The crime scene would be closed up and the cops gone by some time late that night I'm sure.
Or, as has been suggested, one of them texts or calls a brother or a friend or whatever. Impresses upon them in as few words as possible how unless they want me/us to be in prison for years, get your ass out here and pick up this shotgun we put in the bushes, or whatever.
Again, I realize I'm very deep into conjecture land here but I just feel that those who are acting like there is NO WAY something like this happened could benefit from having it laid out in detail how it COULD have happened.