There are two types of magazine springs and their specific applications, coil / round, and box or rectangular shape. The round spring, with the help of a guide, compresses into a hole or socket in the bottom to allow you to load eight rounds. The box spring fits neatly between the front and rear spine taking up all the extra room so it cannot bind or get out of place, a common issue with the coil spring? As the box spring has no hole to go into, it sets flat on the top of the mag bottom and we need to trim the top of the magazine bottom to allow all eight rounds to fit. It actually works well and is a worthwhile effort. Without the above mods, the triple K factor mag spring is weak, and gets weaker as soon as you use it. Best to all, til.....lat'r.....GT
