my objects always have a clear line where the texture ends and begins again. I have a feeling it's how I'm unwrapping it, but I don't know any other way. Theres a black seam in the middle of that bullet and also on the other side of the head. Then the square you can see where the texture ends on the edge's. I painted the bullet so its not an image that I pulled off the internet. I painted the uv map using photoshop, so it is an image texture but the texture was made for that uv map. I also added my UV layout there.
It's not a problem of UV unwrapping, it's just the material itself. You won't get rid of the seams entirely, though.
As long as you don't use purely procedurally generated materials (that means no image textures are involved), it's next to impossible to create a UV map without visible seams. You would have to straighten the polygons on your UV map so it fills the whole texture map (granted your image texture is tileable).
If you want to use image textures and want to get rid of the seams, you need to "paint them away" in Texture Paint Mode. Use the Clone Brush, clone a similar looking part of the image and paint over the seam to make it less visible.
"Hard" 90 degrees angles like in your cube example are still problematic. You can solve this by using the "Chinese Box unwrap" instead of "Smart UV Project" , which results in less seams. Then start painting over the remaining seams starting with a very small brush.