Snap On MW120 MIG Welder

If you cannot feed the wire easily by hand through the liner that is the issue. The biggest mistake I see guys do when they install a liner is to try feeding it through the gun too fast with your hand too far away from the end of the gun. Once it gets to the curve of the neck they put a slight kink in it. Once it has even a slight kink the liner is junk. Pull it back out of the gun and lay it straight on the ground. If there is the slightest kink in it then you have discovered the issue.
To install a new liner. First remove the Tip and tip adapter. Then lay the gun out straight on the ground. Do not pre trim the length of the liner.
Grab the liner between your thumb and fore finger and start pushing it into to gun. Only grab the liner 1 inch back from the end of the gun. Once it gets to the curved neck it may want to hang up. If it does only put a little pressure on the liner and snap the gun so it flips around a bit. This will allow the liner to finish going through the neck. After you have it fed all the way through then lock it in with a little pressure from the set screw on the machine end. Then take your tip adapter and feed it onto the liner. Depending on the adapter some will feed all the way down and screw on the neck and some have a shoulder that stops it. It it feeds all the way through then see where the end is of the liner of you hold it next to the neck simulating it being screwed in all the way. Cut it there with a pair of side cutters. If it has a shoulder in the adapter then slide it on the liner and hold mark the outside of it so you know how far in the liner goes. Then hold it along side the neck and cut the liner at that point.