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I've turned some projos for larger diameter stuff from .50 cal to 25mm for an anti tank gun. I did quite a few for a 20mm anti tank rifle. All in all they work quite well but using pure copper is not a good idea. It has some problems that make it difficult to use. It is difficult to machine because its "gooey" and tends to load up the bit, even in the hardest alloys its a problem getting good surface finish on the parts. It has some problems in use as well with fouling. There are better materials for turning projos and the best is probably bronze. It is much easier to machine, does not foul as badly,has a lower coeffecient of friction, is comparable in density and is a little less expensive than copper(not a lot). I used 934 Bronze in the cannons with very good results. There is also brass which is cheaper than bronze or copper, the easiest of the whole lot to machine (it is the standard by which materials are judged for machineability), it doesn't foul as bad as copper but worse than bronze, lower coefficient of friction than copper but higher than bronze and the density is right there with copper and bronze. I used a lot of 360 brass for projo's of all types. Last one is LedLoy which I have not used but some .50 cal shooters are. It is 12L14 steel which is leaded to make it more machineable. Its softer than your bore but hard still. I can't make myself believe it won't drag the rifling out the bore with it but some swear by it.
There is a good article on Dan Lilja's website concerning the use of these and other materials for bullets in .50's. Lilja is a barrel maker in montana. I don't have a link to his site but google should. You are looking for articles on his site.
Hope that helps
Frank
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