OK, so I bought some mixed stamp range brass in 10mm from a guy. Some of the cases are Sig but they are sort of a brownish bronze color. Are these brass? steel? aluminum? Thanks

I’m pretty sure all Sig ammo is brass cased. The magnet test will rule out steel and I’ve never seen brownish bronze aluminum cases. They are normally whitish in appearance. Brass can become tarnished and the powder darkens it also.

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USS Brewton FF1086
SWCA#3597

I run all my cases that I find at the range with a magnet, you will be surprised how many cases are metal, coated with brass.

Not magnetic, bright and shiny...not tarnished.

Sig labled brass is one of my favorite to load. You can load those things over and over and over. I have about 300 Sig 10mm cases that have taken some mighty mighty loads and are still ticking right along.
I've only ever seen brass from Sig but who knows. Its probably brass.