You have some serious normal errors on the barrel, also seems extremely thick in general.
I think It's a smooth edge issue from Maya. The normal map helps hid it but not completely. Not sure if it would be acceptable to add control edges to make it go away, but in doing so would raise the poly count.
You should harden edges that are about 70* or more, otherwise the normal can't handle the angle, I would guess that is your problem. Control edges wouldn't be acceptable in my book for a low poly model, they should only be used to minimize wavy patterns when baking and then removed for the final low poly.
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