Aside from the actual configuration difference and how the needles line up, a round liner and a magnum also give different effects when tattooing. The above photo are seperate examples of a round liner and a magnum used for eyebrows and also used on a practice pad. I've used the same pendulum motion technique and the same machine speed on the practice pad to demonstrate the different effects of these needle configurations.

Liner needles are typically grouped in a circle and the needles are soldered tight together.  When used for a pixel technique they can give a textured effect especially a 3 round liner for example.  A single needle can give texture as well, however depending on the technique used, you can also get an airbrushed effect similar to a magnum from a single needle, it just takes time to build up the color. Liners give you the opportunity to leave untouched skin space where there's no to minimal color deposited in the skin and also are great when you...