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.