Barking at people in public

If I take Abrams anywhere, as soon as I let him out of the vehicle and he's excited, if he sees people he starts to bark at them. This is extremely annoying and people see a large brown dog on a prong collar barking extremely loudly and "aggressively". Not so good. Once we actually get going and start moving he typically shuts up and stops barking at people that we come across, but he always does it when we first get out of the car. Today he barked at three kids and scared the crap out of them. Luckily when I said, "I'm sorry guys, he's friendly, he's just really excited right now and likes to be noisy" they were like, "Oh, okay!" and didn't care anymore. One kid even tried to strike up a conversation with me and told me his uncle had a dog just like him. Lol

His bark isn't his happy bark, it is his watch dog bark, and his body language is very forward. He acts the same way when visitors come to the house. If he is actually able to greet the person he turns into a mushball, but keeping him physically restrained away causes him to bark and act out. So maybe it is a type of reactivity?

If I have him sit, he continues to bark. (I haven't tried a down, but I imagine it'd be much the same.) If I try to get him to not look at them, he throws a fit. If I shove treats in his face, he'll stop barking and take the treats, but he'll continue to growl.

I may try a gentle leader, to help redirect him? That way I can control his face versus just his neck? Are there any other exercises I can do to help him calm down? As I said, once we actually get going he's fine and doesn't care about people, it is just getting him out of the car that is the issue.

ETA: I've never given him a collar correction with the prong aside from when he's being a dingbat and tries to pull me over. (To which he responds quite appropriately.) I have it on him purely for control purposes. I've been avoiding using a collar correction for the barking incase it is some kind of reactivity issue, although today I was about fed up and was tempted.

What about teaching him another thing to do when he jumps out of the car? I'm not sure if it would work the same way, but because Meg can be so reactive to other dogs and is frequently released from her crate in crowded areas at trials, I taught her to jump up on me as soon as she comes out of the crate. It redirects her from "I'm free, what's out here, where's that dog that walked by my crate a few minutes ago I'm gonna eat him" to "Out of the crate, paws on mom, get a cookie".

With Gusto, although he's not reactive, I wanted him to have a similar behavior that would keep him focused on me coming out of the crate. He releases onto a tug - either an actual toy or his leash - and can keep tugging on it to get out of crowded areas. On the rare occasions I haven't had a leash or toy (getting out of the car to hike for instance) he has sometimes just grabbed my sleeve. Not ideal, but the behavior is definitely trained!

Releasing to a tug, or hand touches (left/right/left/right cookies!), or any other simple behavior that he likes and that requires him to engage with you, rather than his environment, all might help.

I think the hand touches (or similar) are a really good idea. Teach him to wait when you open the door, then release straight to hand touches. My first thought was to put him back in the car when he barks, but since it's likely a frustration issue it might just make it worse.

Hand targets are a good idea! And while he doesn't really tug, I bet I could have a small squeaky ball and use it to the same effect.

I may practice going in and out of the front door, as the only time he goes out front is when we're going somewhere and he gets super excited. That may be a good start point. Then transfer to the car.