A magnum is a tattoo needle with two rows, one on top of the other. They are stacked as you would stack soda cans laying on their side, four on the bottom and three on top for a seven magnum. The two rows are spread apart more than a round so the pigment dispersal is a little smoother. It would be like comparing a fine point pen to a magic marker. The little detailed stuff is for the pen while the magic marker is for larger jobs.
The magnums are made by soldering your needles in a row and laid out flat. Solder at the back, next use a single edge razor blade to weave the needles. One on top, one on bottom, then one on top, etc. Then with the razor in place another layer of solder is applied to hold this configuration. A stacked magnum is where they lay four down and solder them, then lay three down and solder those. Then solder the two layers together. This would be make a seven magnum stacked. Because they are closer then they are smaller, so a nine magnum stacked will fit in a seven magnum tube tip.
Proper use of a magnum is at an angle. You want the magnum to lay flat on the skin, but lay it down so it can penetrates at as slight angle. If you try to use a magnum straight on the skin like holding it will pinch the client and stick in the skin causing the needle to jump. If the needle is flat on the skin then you need to elevate the eye loop till you have about a forty degree angle. You should drag a magnum, not push it. What this means is that you should pull the machine away from the tip of the tube, similar to driving a car in reverse. If you push the magnum forward it will have a tendency to go under the skin making it difficult for the needle to retract and move forward again.
Solid coloring with a magnum is very similar to coloring with a lining needle. You want to color in the circular motion but due to the magnum being wider like a paint brush you cover more area faster. Think of using a magnum like using three lining needle side by side to color. If you go slower then you will cause more damage to the skin. Pay attention to the texture of the skin by wiping away all the extra pigment while you tattoo. It takes practice but you can turn a magnum on its side to use the edge needles like a liner. This will make filling in close to the line work a lot quicker.
One way to practice using a magnum needle is to buy one of those large black markers that have the tapered tip. Practice using the marker at a flat angle to make a bold line, and then turn the marker to get a finer line. Just remember to go backward to simulate needle use.