Some of what you pick depends on what flavor of magazine release that you want built into the unit. So pay attention to this when shopping, & if you can, try out ones with the different types of releases. You also need to decide if you want to alter your action to accept certain types of magazines. As not all DBM's work with stock actions. And do you want the magazine to be semi-protected by the DBM metal or not? Also, some units will accept both AI & AW mags, while others are only designed for AI style mags. Which gives you yet another decision to make. This, along with whether or not you want to have the ability to run magazines that let you run cartridges with OAL's longer than SAMMI spec's. For example, up to 4" in a long acton, or similar options in a short action. Though obviously the OAL won't be 4" in an SA.