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I see on You Tube where they do ammo reviews and then talk about "jacket separation" or fragmentation and it always has a negative implication. My question is .... who cares? Why does it matter? I am sure there is a good reason, I just don't know what it is.
The core of the bullet is soft lead. Really soft. In the most basic terms once the jacket seperates from the lead core there is nothing to hold the shape of the lead core. So the lead can deform or break apart resulting in loss of effectiveness.
Once the jacket is shed the lead isn't "protected" as much as the lead travels through it starts to come apart, depending on the weight of the bullet and the media/material this can happen fast. Because of this the bullet may not penetrate as deep, if its paper no big deal but if defense or hunting you want the bullet to stay together to get to the vital areas. A bullet that stays together/doesnt shed has a better chance of doing this
For big and dangerous game you want penetration. You need to get the bullet into and through the vial area and then some. And hopefully out the other side.
For deer sized you need insurance that if you hit the shoulder blade the bullet keeps going to at least hit the lungs and heart. But even then deeper is better.
For two legged bad creatures it is more of a grey area. If you hit someone with a full power rifle cartridge he is going to have a bad day no matter what the bullet does. Handguns are much lower powered so the FBI says you need 15 inches of penetration (even then most people survive being shot with a handgun). Companies like Glaser got rich in the 90's with their "safety slug" which was meant to fragment on impact and dump all of the energy into the target. I think this has pretty much been discredited today but who really knows.
Once the jacket separates the bullet will come apart/fragment and basically you loose penetration at that point. In a hunting situation this could mean you might hit bone and no vitals and you loose the game. In self-defense, you do not incapacitate because you do not hit vitals or Central Nervous System.
And yet, they all got their animal despite the poor performance...
Most bullets have a velocity range where they are most effective, with poor performance outside that range.
Most jacketed bullets are made with a separate jacket cup and lead core pressed and formed (swaged) into final shape/dimension.Speer used to have what they called "Hot-Cor" bullets. The jacket was heated and the lead core was molten , the core and jacket was sort-of soldered together. Other companies offered a bonded bullet construction.