Can't bake even 1 normal map.

Creepezel

Well, I'm out of ideas, my normal maps end up looking trashy all the time. 
I have this one gun for which I'm trying to bake normal map:
 
Using details from higher detail version of the low poly mesh (not all the details are done yet because even these few, can't bake properly):

But after baking in Blender 2.83.1 results look like this:


I've made sure to keep these 2 meshes in the same spot, even made a cage mesh but here are always some seams and these 2 squares are, for some reason, being stretched to the right. I've tried baking normal map from high poly mesh in Substance Painter but, no difference. I don't know anymore, for some reason I just can't bake a damn normal map. Here's the .blend file so you can look inside by yourself. Hopefully someone else could find out what's going on there.

FrankPolygon

Hard edges that aren't backed up with UV splits will result in seam line artifacts along the edges. Other geometry related issues can cause the skewing and gradation that appears elsewhere in the baked normal texture. Here's a link to a post where I list some resources for identifying and resolving normal artifacts: https://polycount.com/discussion/comment/2730233/#Comment_2730233

Kanni3d

Ontop of Franks suggestion + link, it also helps to rotate your uv shells to be snapped to the pixel-grid, which will alleviate a lot of the jaggies + aliasing going on ( Some of your shells are rotated, and not perfectly straight).

Also worth mentioning that your high res can really benefit from having wider/fatter bevels, just to get more information onto your lowpoly as well.

Creepezel

I will take this into account. Well, gotta keep trying then.