A book about aliens living on Earth who look like humans

Question

I read this book when I was a teenager that I can't remember the name of.

Here are all of the details I can recall

A kid on the cover wearing a metal gauntlet on his arm that extends up to his forearm. In the book, it has some function that shoots bb gun pellets or something similar. This might be a training tool for younger aliens, and a more advanced one is an energy weapon.

If I remember correctly, the story is about an alien species living on Earth as refugees. The kid finds the gauntlet in a shed and starts using it, and he starts to transform? I think the adults in the species are keeping their true origins a secret, but they might be crustacean-like and have crab claws for an arm or something.

At the end of the book, there is a battle with the aliens fighting humans and possibly a spaceship they might leave on.

I believe I read this book around 2002. I think it had been recently published as well, but I am unsure.

Answer 1

The Dark Side of Nowhere by Neal Shusterman as per Story ident : Alien scout-ship crashes (accepted answer at Young adult novel with secret alien society):

The kids (not the adults) have to take monthly shots to keep looking human (because their bodies reject the alien dna as they grow), it fails as the kids go through puberty and they won't be able to replace the disguise because the original dna is lost.

The main character, Jason Miller, lives in the small town of Billington, and has a feeling of not quite belonging. One oddity of the town is that a lot of the families have to receive regular shots which they're told are to ward off a particular illnesses. After a security guard he's befriended gives him a glove that can shoot ball bearings, and tells him to keep it a secret, he befriends other kids in town as they try to figure out what's strange about the town they live in. As it turns out, a fair amount of the town is descended from a crashed alien ship and the shots are necessary to maintain their DNA, which was extracted from people killed in the initial crash (this is part of what begins cluing the kids into there being something very wrong when they realize that antique photographs from the old town seem to be of them). It all culminates in Jason and the other children having to make a decision as to whether they identify more with their planet of birth, or their planet of origin as things come to a violent head.

A copy can be borrowed via the Internet Archive if you want to verify the details.