Maybe one of your UVs is folded onto itself, or maybe you forgot to set the lightmap index.
Though the bigger issue here is how many shells you have in your lightmap. The goal is to have as little amount of seams, and as few amount of shells as possible. If this is a barrel I assume it is 1, 2 or 3pieces, right? If so than that is how many shells you should have in your lightmap. If this object was working and you baked lighting to it there would be so many dark seams it would be unusable. Unless you cranked the resolution way to high.
Check out this doc that goes further in depth into creating proper lightmaps. (Your issue is towards the bottom, But I would suggest reading the whole thing.)
Hope that helps!