One of the most popular modern sporting rifle platforms is based on the M4/M16/AR15 series of rifles. These all have a NATO STANAG 4179 magazine well that accepts a double-stacked detachable box magazine into the action, providing rounds for the firearm. Most modern magazines are 30-rounders with some legacy 20 round and 40-round magazines out there. However, with this as the standard, there is a growing movement and availability of ultra-high-capacity magazines offering 50-100 rounds or more. They all offer an increase in 'ready ammunition' which ideal in extended range sessions or in a possible high-density threat incident. Having more ready rounds allows the shooter less exposure by eliminating or at least reducing the amount of time spent exchanging magazines in a tactical situation.
Let's take a look at these.
The Beta C-mag
Exploded picture of a Beta C-Mag, Betaco photo
For generations of shooters who wanted more than 30-rounds of 556 candy, the most popular option was the Beta C-mag. Designed with two drums built to the left and right respectively from the base double stack feed, the C-mag resembles an oddly shaped boat anchor in profile. The concept is very similar to the 75-round saddle drum of the old German army Maschinengewehr 13 light machine gun popular with early panzer gunners in World War 2.
C-mags have several benefits. I have shot through them extensively and have to say that feed and function overall are acceptable. With the twin drums, it is possible to obtain a realistic prone position with an AR-platform while still having an immense magazine capacity. Most current models include a clear rear cover that allows fast and accurate viewing of rounds available. They have been used by military and law enforcement since 1989, showing that they have been around and been approved for more than two decades.
These magazines are not universally popular for several reasons. The most problematic issue with C-mags is in the loading process. Use of a specialized loader is often required to fully stack these beasts and it is a time consuming process. Many users find that you have to use dry-lube extensively while loading to keep the magazine functional. Others complain of balance issues and a change in the firearm's center of gravity from shot to shot, affecting accuracy. In addition, these guys are somewhat expensive. Used models can sometimes be had for about $200 while new production runs closer to $300 or more.
Korean-made knock offs that appear very similar to these classic magazines are made by such firms as SGM Tactical, PW Arms and Mag-Systems. They run about $150 new but I do not have any feedback as to their reliability or warranty.
Surefire quad-stack mags
Surefire, whose tactical lights and accessories are household names, introduced these big-bodied magazines a couple years ago and they have fast gained in popularity. Magazines of this type are referred to in high-speed circles as 'coffin mags' due to a profile that resembles a classic Egyptian sarcophagus. Surefire construct two variants, a 60-rounder and a 100 with a hard coat anodized aluminum body, nylon followers and cadmium-coated springs.
Surefire 100-rounder coffin mag. The 60-round version is much more maneuverable. Photo by Surefire
Specs from Sure Fire's website are 60-Rd 8.7" (22.1cm) tall, 1.66" (4.2cm) wide at base. 6.4 oz. (181g) wt. empty, 2 lbs. (907g) wt. fully loaded. 100-Rd 12.2" (31cm) tall, 1.66" wide at base. 9.6 oz. (272g) wt. empty, 3.3 lbs (1.5kg) wt. fully loaded.
These coffin mags are more affordable, at about half the cost of a C-mag. Furthermore, the 60-round version has about the same profile as two standard 30-round magazines speed-taped together.
The only problems reported from range analysis are that in my experience the last few rounds will not feed. Other than that, they feel good and closer to a natural weight balance than double drum mags. In addition, it is understood that shooting an AR-platform with a 100-round coffin mag in a prone position is a no-go.
Magpul has patents on what they are calling their 4x2 quad stack, a coffin magazine of a similar concept. Announced almost two years ago, it has not hit the market yet but it is still on the horizon.