The MIG liner is a much more critical part of the MIG process than is generally recognised. The main requirement is low friction but useful life. A lot of feeding issues can be down to the liner needing to be changed as debris can build up and start to intermittently jam the wire. Teflon or graphite/polymer tubes are used for materials like aluminium and MIG brazing wires but not regularly for steel wires as the steel wire wears them out quicker.