It doesn't _always_ work. I don't think anything ever will. I'm not even sure the EFMJ will expand better/more consistently than other JHPs out there. It does not in water jugs through denim.
The downside: fired some 165/40 EFMJ from a G23 at 970 fps into jugs of water through denim. Went to 5th jug, minimal expansion of .425! Some made it to .53, but some exited 5 jugs and were not recovered. For comparison the lowly PMC 165/40 JHP at 985 made it to the 4th jug and expanded to .66!
Pulled some bullets, pumped em up about 130 fps to 1100 (7.2 gr PP), and through denim some made it into 4th jug at .60, some still exited all 5 and not recovered. Have no idea what's up with that.
BTW, the best so far is the PMC Starfire. through 6 layers of denim the 180/40/975 did 3/.75, the 155/40/1125 3/.81! The HS, GS, GD didn't even come close.
Also heard from somebody who shot through gallon jug of water into wooden boards (treated 1x6). The 200/45 +P EFMJ expanded to .80 but just dented the board. A Win JHP went through a couple baords and stuck in a 2x4.
Then again, jugs aren't gel, and gel isn't people.
It is accurate, reliable, clean, and easy to handle w low flash. All of which may have more to do w "stopping" than how big it does/doesn't get?