FWIW, I have an Echlin rotor and Echlin cap with copper contacts, with just a few hundred miles on them. Last week it developed on occasional miss at low RPM (<1500). A few days later, on a longer club run, the miss became more frequent and more pronounced.
When I opened things up later, I found that the inside of the dist. cap was coated with what looked like a fine spray of copper particles. I cleaned them out thoroughly and the miss is no more.
Not sure it's the cap's fault. Noi sign of mechanical interference. Rotor looked fine. I have a Crane XR700 ign. with a Crane coil (PS20). I've seen comments on a couple of TR boards about crossfiring and other problems when going to higher energy ign. systems, but I'd never seen anything like this before. Just thought I'd throw that in.
Tom