These magazine extensions are constructed of blued solid steel and come with spring, base clamp with swivel stud (included) to work with quick detachable swivel (not included) and a high visibility follower (some extensions require factory follower and will not come with one). The threaded tube replaces you magazine cap and gives you the ability to add extra rounds to you magazine. Easy to follow instructions are included. The magazine extension tube is great for home protection, competition shooting, and law enforcement. See chart below for the make/model and the magazine capacity.