So I went out shooting with some buddies the other day, and one was shooting a Remington. Myself and my other buddy both shoot Savages. I've always noticed black residue around my case necks on fired brass. My buddy with the Remington was shooting was shooting Federal GMM, and I put one round of it through my buddies Savage to see what his gun felt like with the brake he just added. I ended up with a box of brasss, and I noticed the brass from the Remington was much cleaner around the case necks than the one I shot from my buddy's Savage, and that's also the way my stuff always looks...

What causes this?

The Remington was a Rem. Custom Shop 40-XS
Savage was a 10FCP-SR I think?
And my gun is an Axis

These are the same two cases, I just rolled them to get a full circumference view. The light washed it out a little, so it doesn't look as dark, but you can still see the black on the case neck on the right.