It can. Not all copper is the same.
Bill Steigers of Bitterroot told me he had to reject as much copper as he accepted,because it did not meet his specs.
I'm no metallurgist,so can't say how it differed but apparently some stuff was not as malleable as others.It can affect how the bullets behave,expand, etc.
I would imagine that copper, like many other metals, can be "100%" pure by some definition of purity, while yet having .04% - or whatever- of something which might affect the character of the metal. Certainly things like cartridge brass can be affected by the simply by the presence of fumes of materials like ammonia. Granted, brass is an alloy, but similarities might apply. Trace materials can be very damning when it comes to chemistry. And then there are factors like heat treatment of the metal. There is no easy way to harden copper alloys that are dead soft without cold-working them. Coils of copper wire that are supposed to have a specified hardness might be quite unsuitable if they are delivered in dead-soft condition. Then again, wire that is delivered too hard could be annealed, but that would add more expense. It might also depend on the entire batch being consistent throughout to start with.
