Glock 9/40 Mag Extensions

Roberto G.

Cain Arms 9/40 Magazine Extension Review


This review will be split into several sections that encompass varying aspects of this product. I purchased one magazine extension and received another to review.



The Cain Arms 9/40 magazine extension is a polymer based magazine extension that will add either 5 or 6 extra rounds to a standard Glock magazine. The product is installed by removing the Glock baseplate from the magazine and discarding the plate, keeping only the main spring housing and the spring. Proper installation calls for slight stretching of the spring in order to compensate for additional space that will be introduced into the Glock magazine—yes that’s right, you only need to purchase the magazine extension, no need for aftermarket (perhaps unreliable) spring kits. Once the spring is stretched, the Cain Arms extension slides onto the bottom of the magazine where the base plate once was. Once slid onto the ramps of the magazine, an additional layer of security lies in the presence of a screw that is screwed into the magazine extension. Fits moderately flush with Glock magazine well if used in the appropriate handgun (i.e. a Glock 19 magazine used with a Glock 19).


Initial Thoughts:

The market for magazine extensions is saturated with metal extensions of varying additional capacities. The installation was simple enough that someone without any serious firearm experience can still install the magazine extension. The magazine extension had a tight fit to the magazine, no observable wobbling of the magazine extension and no gross defects in manufacturing. Based on my installation, I was able to achieve a +5 additional capacity in my Glock 19 magazine that fed every round out of the magazine efficiently and without failure consistently. The use of standard Glock magazine springs made me confident in the fact that the long held reputation of Glock reliability would not be faltered by the use of an aftermarket part. My main concern was the elephant in the room, that is, the use of a polymer magazine extension. Pre-Glock era of guns had an adage that, “Why would I want a piece of Teflon defending my life?” whereas now it seems to hold, “Why would I want that unnecessary weight of an all steel 1911?” My fears were silenced upon receiving the product. The polymer used and the manufacturing process of 3-D printing allows for a magazine extension that is rigid, solid, and not able to be wore down by normal processes, granted a belt-sander would surely wear down this product, but in the “real world” such events are not possible, leaving this product as durable as the metal magazine extensions.



As stated before, the big issue of concern is that the magazine extension is polymer, which is susceptible to heat damage, wear, imperfections, and in general being unaesthetic. The product has slight striations in the polymer, that act as a grip, similar to sandpaper grips that many install on their firearms, granted not as severe, but it is something much appreciated as smooth magazine extensions can be slick, even more so under sweaty conditions. The finish to the magazine extension is flawless and Cain Arms custom color option holds true to it’s statements, I ordered a royal purple magazine extension to match my custom purple Glock trigger—the color was spot on and I even received an email with a sample prior to any production, a service that not many companies offer. This magazine with the magazine extension has been left in full capacity (a total of 20 rounds in a Glock 19 magazine) in my glove compartment where temperatures in central Texas (during the time of this review (March)) are a crisp 70 in the morning, 95 in the afternoon, and the night can dip down to low 60s. Throughout the 7 days of temperature fluctuations there is no observable change in the polymer nor is functionality affected.


Tactical Application:

In a world where polymer guns are dominating the market, it only makes sense that gun accessories and parts will soon follow suit, the long held belief that it is “Teflon” has long been disproven and grants a slight, but noticeable weight savings. Use of polymer can potentially save ounces in weight when compared to their metal counterparts, and in the words of many a tired LEO’s and military members, “Ounces equals pounds, and pounds equals pain” this may provide a beneficial alternative. Additionally, the polymer used, in conjunction with it’s texturing, and minimizes the amount of light reflected, allowing for a more concealed accessory than compared to its metal counterpart. The ability of these magazines to be offered in solid, standard colors in order to match a black duty uniform, as well as custom colors really allows the user to attain a magazine extension that can potentially blend seamlessly into their kit.



This product has left me very satisfied and hoping that Cain Arms will continue their work in polymer accessories, granted there is always room for improvement so, my 2 main complaints are:

1)      At the bottom of the magazine extension, there is a hole that seems (at least to me), no practical applications. This hole in the bottom only leaves space for debris to accumulate and if used for anything beyond range use can literally make the difference between life and death.

2)      The variability that comes with installation leaves much room for error. The fact that “stretching slightly” to me may mean a world of a difference from you is worrisome. One negative I’ve noticed after being loaded to full capacity is that my magazine no longer stays locked back upon shooting the last shot which is inevitably due to my incorrect stretching of the Glock spring. Regardless of that, each round feeds properly and I have yet to receive a failure to feed.



This product has left me very satisfied, its low (relative) price point is very appealing and it’s use of OEM Glock magazine spring leaves no worry of reliability issues. I am carrying this magazine (with the Cain Arms magazine 9/40 extension) as a backup magazine and trust my life with it. As with the firearm world, polymer is the future of the gun community. I highly recommend this product. 

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