I wanted to post a lil trick I found for getting that Mig (Aluminum) wire out of the copper tip if it burns back through.
First off, I've been using an 0.35 tip for 0.30 wire (no problems) but if you get a burn back I found that you can do this if it's not too bad. Cut a piece of 0.35 Er70S6 "wire for mig welding steel w/gas", (copper looking wire) about 3/4 of inch long and put your tip in a vise upside down put the wire in the tip and make sure the wire only sticks out about an 1/8 of an inch then hit it with a hammer. A lot of times it takes a few tries but the aluminum should pop out. Yesterday I went over doing this and I saved about 9 tips, thats about $15 bucks worth.