Mig Welding Gun Liner:

Inside the whole length of the mig welding gun is a mig liner. The liner is a length of steel wound conduit that is fitted into place and has the welding wire running inside of it. The steel liner looks like the same stuff that they use in flexible drive shafts for whipper snippers, and it also looks the same as the flexible white plastic coated springy cord that you can hang curtains from.

Liners are available in different sizes and lengths. The deal is that you buy a liner that is as long as, or longer than your welding cable. If it is longer than you need, you just use some pliers and chop some off the end. The liner will travel starting from the welding machine euro connection (or whatever connection there is) to the back of the contact tip holder. The firmness of the steel liner will allow safe passage of the mig wire and help prevent feeding issues.

You can also get plastic or teflon liners. These are used for when you have to feed aluminum wires. This is because the wire is very soft and the metal liners have more friction in them than the plastic ones. Some brands and models of welding guns will have a two piece kit.