When I was a kid, about 8 to 11 years old, I saw this anime about a teenage boy that can transform his arm into a giant cannon. There's this antagonist that eats a whole human from that anime and he also stole their abilities after eating them. The antagonist can also transform his arm into a cannon but much bigger than the main character.
3 Answers
The anime you are thinking about is The Law of Ueki.
The Giant Cannon is "KUROGANE" (Sorry it's still so cool to say it), his Sacred Treasure.
His power is basically to turn Garbage into Trees. You might remember his Cannon being Tree-ish from the bottom.
The antagonist (Hanon), eats humans and keeps them inside his body and absorbs their powers.
This sounds a bit like Trigun, which was aired in the US on Cartoon Network in the early 2000's on Adult Swim. I don't remember the specific age of the protagonist, but he portrayed himself very childishly to keep other people off guard. In the final(?) episode, he fights with his brother, and they can both turn their arms into organic guns.
And, as luck may have it, Funimation (who own the distribution rights in the US) has put the entire series (in Japanese) up on youtube! Here's a clip of the protagonist (in red) getting his arm turned into a gun:
I thought it was Space Cobra. Maybe Trigun
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1Could you edit to explain why you think this might be it.– TheLethalCarrot ♦Commented Nov 18, 2018 at 22:21
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2In addition to what TLC said, you might want to split that into two answers - one for Space Cobra and one for Trigun. This way people can vote on them separately, and only the right one (if it's one of these two) gets accepted.– JenayahCommented Nov 19, 2018 at 0:10
MegaMan?