Bullets that are slightly flattened on one side by the military’s bullet-puller (used for de-milling surplus ammo) make things exciting. When the seating die is crimping the case neck around the bullet, it can make it non-circular.
This causes rather amusing chambering issues, like when I tried to load and fire my very first shot from my new DPMS AR today. Some swearing ensued. A total of three rounds (out of the ~150 fired today) were out-of-round enough to cause a stoppage.
Moral of the story: surplus bullets are great for range trips and fun, but do not depend on them for anti-zombie use.