I was recently at a store looking for some ammunition I needed when a young new enthusiast approached me and asked, “What does FMJ stand for?” I politely told him that FMJ stands for Full Metal Jacket where they coat the outside of the lead bullet with a harder metal such as copper or brass. That then prompted him to ask a follow up question, “Why do they do that anyway?” I knew this was an opportunity to impart some knowledge and provide a little bit of education.
To be truthful, I did not go into as much detail there in the store as I could have, I just gave him the short answer, given that it really was not the proper forum. But in so much as this might be a more appropriate forum; I am going to start at the beginning.
The birth of our country was won in a war we call the American Revolution. We prevailed over soldiers of the English army using long rifles that used black powder to propel lead shot balls toward their target. Black powder was the technology of the day and it served its purpose. In the mid 1800s we began developing cartridge based ammunition that encapsulated the gun powder in a metal casing that held the lead shot as well. It revolutionized firearms making their use much easier and faster.
Then, near the end of the 19th century, smokeless powder was developed. There were many advantages to the new smokeless gun powder, first and foremost is the extreme reduction in the amount of smoke it produced. But along with that, smokeless powder generated much more perceived power through greater pressures.
Suddenly, the same lead projectiles that used to leave the barrel at 800-850 feet per second were traveling at 1100 to 1500 feet per second and more using the same amount of powder. Muzzle energy and impact energy was nearly doubled.
One thing that was discovered as a result was that when muzzle speeds approached 1100-1150 fps and more, it actually was so fast that lead bullets would begin to melt from the friction as they traveled down the barrel. That melting would cause small particles of lead to be deposited in the barrel. As more and more lead residue was left behind, the accuracy of the rifle was greatly reduced.
So in an effort to keep the lead bullets from melting, they came up with coating the bullets with another harder metal that would not melt so easily, such as copper or brass. The bullets were then “jacketed” in another metal. Fully covered it was a “Full Metal Jacket” or FMJ.
This allowed manufacturers to develop more powerful ammunition with muzzle velocities of 2000 -2500 fps. Many rifles now are producing muzzle velocities approaching 4000 fps as with many .223 rounds. This would never be possible without a metal jacket.
So after all that, FMJ simply stands for Full Metal Jacket, which means the lead bullet has been coated with a jacket of a harder metal, usually copper or brass.