I'm no expert, but I have soldered Kimber hook-up to Cardas copper posts recently. The one thing is, it takes a bit of time to get the post hot enough to melt the silver solder. Put the tip on the post and the wire and leave it there. I keep touching the solder to the post and the wire cause that helps to conduct the heat to the post. Once I see that the solder will melt on the post only, I go for it. I don't know what that sideways hole is for. I wanted the wires to come straight out of the post, so I didn't use it. One other thing....Cardas and some other wire, I assume, has a coating on it that must be removed before solder will stick to it. Otherwise the big thing is getting the post hot enough. If cardas solder is like Wunder solder, the flux is in it.