In our opinion these are the only correctly made mag extensions on the market. Other magazine extensions on the market are simply rectangular blocks. If you look at your magazine the bottom is angled so when you attach a competitors extension, it is not parallel with the column of cartridges in the magazine. This probably works fine for +2 rounds, but it would not work for much if any more. We decided to do it right, and cut the attachment rail at an angle to the pocket in the mag. This means that the column of rounds travels straight down into the extension. This took extra time on the mill and we chose to cut them in one operation on the 4th axis, but it was worth it to deliver to you the most advanced product of its kind. The fluting around the sides make an excellent gripping point for mag changes and also helps to remove some weight. After starting with the American Rifle Company +4 mag extension, we realized that with a little bit of redesign that it could work equally well with AICS an Accurate mags. The 419 universal mag extensions are designed to work on a variety of magazines from the ARC 10rd mag to AICS 5 or 10rd and Accurate Mag in either standard bolt face or on SAUM/WSM mags. They are NOT COMPATIBLE with AI AW magazines, as those have a fixed/welded floorplate. Easy install, great performance, and the quality you’ve come to expect from Area 419.