A full metal jacket bullet has a soft core (most often lead) that is encased in a separate outer shell made of harder metal.
Bullet manufacturers have their own propriety methods for creating full metal jacket bullets. How a bullet is jacketed and the materials used to produce it affect how it will function once fired.
Some of the common types of full metal jacket bullets are:
FMJ ammo performs well ballistically — it doesn't lose speed or accuracy once fired.
The casing on FMJ bullets enables them to feed reliably from magazine, to chamber, to ejection. Also, the outer jacket helps prevent FMJ ammo from leaving deposits in the barrel of the gun.
Rifles, revolvers, and pistols all have rifled barrels. When a lead bullet moves across rifled barrels, small bits of lead are left behind. Over time, lead can build up and cause shots to be less accurate. With a full metal jacket bullet, the lead core is covered by a harder metal thus the amount of lead left behind is greatly reduced. With less lead left behind, more shots can be fired before accuracy decreases.
Because of the way FMJ bullets are made, they do not expand once they hit their mark. Therefore, they are more likely to go through an object than a bullet that does expand (such as a hollow point).
The tendency of FMJ to shoot through an object can be a marked disadvantage for hunters or people wanting a firearm for self-protection.
While you must ALWAYS be aware of your target and what is behind it when shooting, this is especially important if you are shooting FMJ ammunition.
Full metal jacket bullets are known for their smooth performance and reliable feeding when used in semi-automatic firearms.
Because of this they are often the ammo of choice for competition shooting, target practice and plinking.