Scale edges inward evenly I don't know which title suit my question the best. But essentially what I'm trying to do is to create an indentation inward for each piece of wood that encircle my barrel. The only solution I find so far is to manually edit every single edge, but there must be another way to do this for all the edges at the same time.
So the first image is what I need the edges to look like:

Here you can see how the edges look like without my manual scaling:


So as you can see from the first image, I need to scale the edges inwards themselves(?) Is essential for me to create this kind of indentation but each edge has a different angle so when I scale them I get a crazy result.
Also, I should apply this for each edge that encircles the barrel.
The barrel is like this:

And this is the shape of the edges:

I hope I was able to explain the problem I have.
I really hope that you can help me find the solution, I don't use Blender often, I'm a beginner and sometimes I feel overwhelmed.
Thank you in advance for your help.
EDIT: I'm doing this operation while using the mirror modifier on the Z-axis
The shrink/fatten tool is exactly what you need! Select the middle loop of each of the barrel's sides (the one you want to make slightly smaller) and press Alt S. This will shrink the edge loop equally in all directions, giving you the shape you're after.