A jacket wraps around a bullet’s core, keeping its shape together, and keeping the lead contained. A jacket also helps interact with the rifling of a barrel and get the projectile spinning and keep it accurate at longer ranges. A bullet’s jacket is a critical component, as it usually determines what you use the bullet for. For example, a soft point round has a partial jacket, leaving the lead exposed at the tip. This round will hit something and mushroom out from the exposed lead tip, meaning it would be a good round for imparting a lot of damage on target and knocking it down quickly. Another example would be the full metal jacket round, or FMJ, this round has almost no exposed lead (FMJ rounds often have exposed lead on the backside, where it won’t affect impact or accuracy) and so it will usually hit its target and simply blast right through it, not expanding at all.