Underpowered bullets.
Because Black Lives Matter®
Cannot be “underpowered”. If so the weapon will not cycle. Don’t forget, it is the powder in the cartridge that generates the energy to push the slide back and eject the spent casing.
No, I suspect the problem is in the hollow point bullets the cartridges are loaded with. My guess is they are fragmenting upon impact with any resistance. That Carhart jacket may have been just enough.
To me, this story makes a good case for ball ammunition. Unjacketed. Give’em lead poisoning.