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Bullet jacket composition does vary, not only by manufacturer, but with different lots of jacket metal used by the same manufacturer. Places like the FBI labs try to keep up with those variations but usually can't. At one time, the FBI analysts reported in many cases that bullet x (the crime bullet) came from box y (found in the possession of a suspect).
The problem was that they were finding what they wanted to find and finally admitted that they just couldn't make that determination except in a very few cases. That caused a big hoorah and some convictions obtained mainly on bullet jacket evidence were overturned on appeal and retrials ordered.
Markings on the bullet caused by the rifling in a barrel are on much more solid ground. Like fingerprints, they are unique, even from barrels made at the same time by the same factory. Even that is not 100%, though, since the bullet can be damaged by impact or damage to the gun (accidental or deliberate) between firing the crime bullet and the time a comparison can be made can prevent an accurate comparison.
Jim
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