I recently bought a case of Federal XM193A (came packaged in black American Eagle boxes) from ammoman, and was surprised by what I saw at the range this weekend. I know these are loaded to “military spec”, and the previous case I got showed signs of high pressure (flattened primers, extractor marks). But the shells from this recent lot show signs of way excessive pressures. Out of a box of 20, I had one primer completely blown out, and had a few shells with clear signs of gas leaking from around the primers. I think I might pull the bullets, measure the powder, and try and build up a load with a little less powder.
Is your AR barrel stamped 5.56mm? The NATO chambering is to a different tolerance than the SAAMI chambering–or the 5.56 and .223 respectively. XM193A ammo shot in a .223 Remington (SAAMI) chambering will cause dangerous back pressure.
You can safetly shoot .223 in your 5.56 AR, but not 5.56 in a .223 rifle.
Just a thought…
I’ve fired literally thousands of rounds of Federal Lake City XM193 out of a number of different AR15’s I built. I’ve never even once experienced what you’re seeing. I would be highly concerned that you have headspacing issues, chamber issues, or something else with the weapon you are firing these out of.