I came across this brass while reloading the other day. Headstamp reads “IMT 9mm Luger”. You’ll notice on the cut out piece of brass that there’s a ridge on the inside of the case where the brass gets thicker, as opposed to normal brass that thickens gradually down the length of the case. The trouble I ran into with this stuff was when running it into the crimp die, sometimes they would stick. They would stick so hard that I was using both hands on the lever to get them back out. They did seem to be fine after getting them out of the crimp die, but I was afraid that I was going to tear the case apart I had to use so much force on them.
After looking a little online, I found this thread on Arfcom that says this brass is being used by Freedom Munitions. I see a lot of people use this stuff at matches, so I’m surprised that this is the first time I’ve come across this brass. Maybe I just got a few pieces that were way out of spec or something. Dunno.