Apparently those 311 bullets have a steel core, then a soft copper jacket with a steel skin on the outside.
I am guessing that the soft jacket is so the rifling has something to engrave.
A friend in the US sent me the web link and mailed me one of the bullets. At my suggestion, he bought a #4 a few years ago, and now he has some bullets he can load in it.
As has been pointed out here, Widener's can't mail bullets to Canada. I am lusting at some of the cheap rifles they advertise, and will feel a bit better when (if?) that ship arrives from Russia with our SKS's.