EFMJ stands for expanding full metal jacket. This bullet was developed by Tom Burezynski, the same man who invented the famous Hydra-Shok. The idea was to create a bullet that would always expand regardless of what it had to pass through—wood, glass clothing or anything else—before it reached the bad guy's body. This was accomplished by eliminating the hollow point that often filled with material from an intermediate barrier. Bureynski ingeniously filled a copper jacketed, flat-nose bullet with a polymer insert and pre-scored the jacket to collapse, but remain intact. All of the EFMJ bullets I've tested look just like this one no matter what they pass through, and because they expand very quickly, they create wide cavities in gelatin.