Magazines play an integral role in the functioning (or non-functioning) of all firearms, even moreso with a 9mm AR. The 9mm AR was originally designed around a modified Uzi magazine that had an added tab on the back of the follower and a channel in the back of the magazine to accommodate it and facilitate last round bolt hold open (LRBHO), and a relocated mag catch slot.
The gold standard for 9mm AR magazines has been, and still is, the 20 or 32 round Metalform magazine. Prior to 2008 or so, these magazines were pretty much only available as Colt branded product from Colt LE dealers or through the used marketplace. They were/are available in several configurations and can also be found in "pre-ban" status for those of us in states that still have magazine bans in place. These magazines were originally available in a parkerized finish with a metal follower and later were offered in a blued finish with polymer follower. In 2009, Metalform began selling the magazines directly and in early 2010, they discontinued the polymer follower. These mags have sold for upwards of $100 each in the past and Colt marked "pre-ban" mags still command prices of upwards of $50-60 today. Non-Colt branded versions from Metalform however, have been sold at $19.99 and $29.99 from CDNN Investments for 20 and 32 rounders respectively.
Colt/Metalform Magazines - 20 round on left and 32 round on right
Due to the prices of the Colt branded Metalform magazines in the past, a popular alternative was to modify existing Uzi magazines so they could be used in 9mm ARs. The best Uzi mags are from IMI or MecGar and they are usually available for $10-15 per mag depending on quantity and condition. The modification process involves cutting a new mag catch slot and opening up the back of the magazine between the feedlips as shown in the schematics below.
This modification still allows the mags to be used in an Uzi, but would require two presses of the mag catch/release button when inserting or removing mags in order to clear the added AR mag catch. Some users find this to be too much of a hassle and instead prefer the VMHytech mag block, which is the only solution that allows one to use unmodified Uzi mags in a Colt style 9mm AR, but does not allow LRBHO.
Not long after the Federal Assault Weapon Ban expired in 2004, ProMag offered a polymer magazine for Colt style 9mm ARs. These magazines were not very reliably originally and simply didn't work for many people. They have been redesigned over the years and they seem to be a decent mag, but they are prone to swelling when fully loaded which can make them difficult to insert into the gun and prevent them from dropping freely. Some have also had the magazines split, usually along the back seam, rendering the mag pretty much useless. At the price point they are at, they are not really a bargain and one might be wise to consider a more durable metal magazine.
Around 2008, CProducts introduced a stainless steel magazine in 32 and 20 round configurations. There seemed to be some issues and confusion when they were launching them as they were trying to create multiple versions of their magazine for the various mag blocks that were on the market - one for Colt, one for RRA, and one for Hahn and other blocks with various markings - X, XX, ??. This eventually led to some issues between CP and Rock River Arms supposedly because CP's mag required RRA 9mm AR owners to move their block so the CP mags would work. CP wanted RRA to change their product to work with their new mag and apparently RRA wasn't interested in doing that and simply continued to ship their guns with their modified Uzi mags that they knew worked 100%.
Since their introduction, CP touted their mag as being the best, yet it had issues with various mag blocks and continues to have issues today, even though it has gone through numerous changes and updates to improve functionality and reliability. Although CP touted for a time that they were Colt's new mag supplier, displacing Metalform, it would appear as though that was short lived and Metalform was used as a dual source, and CP is no longer a supplier of 9mm mags to Colt as of mid-2010. So, what does that mean? In reality, nothing much. There are many people that uses this mags and report great success with them, and there is nearly an equal number of folks that report terrible issues. The best bet if you want to try CP mags is to make sure you are getting the latest and greatest iteration and to try them in your gun before stocking up on them. Original versions had a gray marlube finish, almost paint-like finish that wore very easily and may have an X or XX on the body. Later versions switched to a black teflon like finish and the newest versions have the black teflon like finish, orange follower and stamping indents near the feedlips that are similar to those found on Metalform mags.
older style (2008-2009) CP mag on left and newer style (2010) CP mag on right
The Beta C-Mag is a 100 round dual drum magazine that was made to work in Colt style 9mm ARs. Some people question the reliability and longevity of these mags as they are mostly polymer and very heavy when fully loaded, but they tend to work fairly well and if maintained properly should provide years of trouble free use. They are one of, if not, the most expensive magazines on the market at around $300 each, but to each his own.
There are numerous other magazines from the likes of National Mag and others that are problematic and just not worth the effort. It is advisable to stay away from these sub-standard magazines to save yourself the headaches they will surely cause you.
Last updated: 08/16/2010