[3] Aftermarket Magazines
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[3-1] Metal Magazines
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The majority of metal aftermarket magazines (Sterlings excepted)are very substandard, using cheap springs, flat floorplates, and black plastic followers. The mag bodies are often dimensionally out-of-spec, being too large to fit in the AR mag well, or will be assembled with one half higher than the other, resulting in the feed lip on one side being too high or low. The feed lips aren't connected to the rear of the mag body like USGI mags are, making the feed lips weak. These mags are either made from steel and are quite heavy, or made from aluminum stock that is about half the thickness of USGI mags. The "finish" on these mags is black paint applied directly onto the mag body. These mags are known for their feeding problems, short life spans, and poor workmanship. However, they cost less than real USGI mags and are widely available, which is enough to attract many buyers at least once. Recently, the prices on these magazines has risen to near-USGI levels, giving further reason to avoid buying the aftermarket magazines at all.

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[3-1-1] Sterling
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Sterling one of several companies who manufactured a version of the ArmaLite AR18. They also made a semi-auto version called the AR180. These rifles share few parts with the AR-15 series, but the magazine design is very similar. The differences are the mag catch hole, which on the AR18/AR180 is a thin slot on the opposite side of the mag, and the follower design, which has a lower shelf for the bolt hold-open.
Many of these magazines also had standard AR15 mag catch holes, and work in AR15s.
Sterling mags are steel, and were available in 20, 30, and 40-round capacities. They are the only 40-round magazines available that used high-quality springs and mag bodies, and are highly sought after. For dedicated use in an AR15, the follower should be replaced with an anti-tilt green follower. In addition to aiding the feeding of the rounds, this will also allow the follower to trip the bolt hold-open on an AR15. Sterling magazines were last made in the early 70's. Floorplates stamped:

By
STERLING England

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[3-1-2] USA
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USA Magazines is the largest and most well-known aftermarket AR15 magazine manufacturer, though they make many other types of magazines as well. USA made 30 round magazines with both steel and aluminum bodies, and also made steel 40 round mags. These mags have no floorplate markings, and have round drain holes in the sides of the mag body on the top and bottom end of the mag.

USA magazines are often misleadingly advertised as "USGI" mags, especially the aluminum-bodied mags. These are quite possibly the worst AR mags on the market, and should be avoided. No brand markings.

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[3-1-3] Triple K
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Triple K mags

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[3-1-4] Western Metal Products
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Western Metal Products

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[3-1-5] ProMag
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ProMag is a well-known handgun magazine manufacturer who also made steel 20-round AR mags. These are some of the few aftermarket 20s that are of the straight-body design. The most common problem with these mags is being too large to fit in the mag body. As long as they fit, they seem to feed and function pretty well. No brand markings.

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[3-1-6] Millet
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There is some speculation that Millet magazines are simply repackaged USA magazines, as there doesn't seem to be any evidence that Millet existed as a company before the 1994 ban. The magazines themselves seem to be identical, and the packaging is even similar.
Avoid these mags. No brand markings.

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[3-2] Plastic/Nylon/Polymer Magazines
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Quiet
No finish
Bulky

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[3-2-1] Orlite
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Orlite magazines were developed for the Israeli Armed Forces. The bodies are made from high-melt-temp black nylon with a steel reinforement mesh imbedded in the top 1.5 inches to strengthen the feed lips. New Orlites come with a rubber cap to help keep sand out of the mag.
Orlites have a rib around the middle of the mag which helps to seal the bottom of the mag well and also prevents the magazine from being over-inserted and the feed lips damaged. The location of the rib was based on the deep mag well beveling on Colt M16s, and in ARs that aren't beveled as deeply, early Orlites won't seat completely. This is easily correctable by trimming a bit of material from the top surface of the over-insertion rib; just enough to allow the mag to seat. This problem only occurs with early Orlites with mold numbers from 0/1 to 0/19. Starting with batch 0/20, the rib was moved about 1/32" further down the mag body, and these later mags work in all ARs with no modification. Used Orlites are often found at gun shows with their over-insertion ribs completely ground off. Avoid these mags, as it's too easy for these mags to be over-inserted, banging the feedlips against the bolt or getting stuck.
Orlites are quite reliable, and were general issue in the Israeli Army through the 80s, but it was found that they don't hold up to hard combat use as well as USGI mags, which Israel switched to in the 90s.

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[3-2-2] Thermold
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Thermold magazines were developed by Master Molder in Wilson, South Carolina, and were "pitched" to the US military as a replacement to aluminum "USGI" mags, but the US military was unable to come to an agreement with the owner. Master Molder then licensed the design to the Canadian military, who manufactured a version of the Thermold 30-rounder as the standard-issue magazine for their military.
Thermold magazines are made from a dark gray polymer and have anti-tilt followers. They have several reinforcement ribs across the lower half of the mag body.

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[3-2-2-1] USA Thermold 30s
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The most common of the Thermold mags, these were made using DuPont Zytel, a fiberglass-impregnated plastic that is very strong and has a high melt temperature. Mold markings:v
???

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[3-2-2-2] Canadian Thermold 30s
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The Canadian military licensed the Thermold design, but did not use Zytel as the polymer, using a less expensive plastic as a substitute.
This was not a wise decision, and the Canadian military had problems as a result. The most famous problem was the feed lips melting when their ARs were fired for extended periods on full auto (especially with blanks), leading the Canadians to dub the magazines "Thermelts". The other common problem was the mags splitting or cracking in cold temperatures or when dropped. After Desert Storm, the Canadian military changed to USGI aluminum mags, which are now standard issue.

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[3-2-2-2-1] Canadian Thermold "Blank" 30s
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The Canadian military made some Thermolds with bright orange plastic as "blank only" mags. These are relatively rare, and are made with the same plastic as the standard Canadian mags.

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[3-2-2-3] Thermold/Master Molder 20s
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Master Molder made straight-bodied 20 round mags. These are US-made from Zytel and are the best aftermarket 20s. Mold markings:

???
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[3-2-2-4] Thermold 30/45
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Among the most unique magazine designs, the 30/45 magazines were designed with an extendable bottom section that would allow the magazine to be fully loaded to 30 rounds and stored (with the base extended) with the spring under minimal pressure. This allowed long-term storage of fully-loaded mags with no wear to the spring or the feed lips. They can also be loaded with 45 rounds when the base extended, though they shouldn't be stored in this configuration.

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[3-2-3] Ramline
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Ramline (now owned by Blount) manufactured a 30-round magazine that was designed to work in both AR15s and in Ruger Mini-14s. The mag body and follower are made from a transparent yellow/brown plastic and the spring is the standard coil-type. Because the mag is longer front-to-back compared to a GI magazine, the Ramline mags are an extremely tight fit in AR15 magwells, and often require some sanding to fit. They tend to fit better in Mini-14s. The plastic used in the mag body is brittle, and the feedlips tend to crack and break over time.

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[3-2-4] Eagle ------- Eagle made a 30-round magazine which they called "Beta" (not to be confused with the Beta-C drum magazine; the companies aren't related).
Eagle magazines are a dark grey transparent plastic, and they have ribs on the bottom similar to Thermold mags. These mags use flat-wound "constant-tension" springs attached to the front and back of the follower, similar to the design found in most high-capacity 10/22 mags. The plastic mag bodies are brittle, and crack easily, especially at the feed lips.

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[3-2-5] MGW
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MGW is best known for its 90-round "snail" drum, but they also make post-ban legal 10-round plastic mags. The polymer used for these magazines is very soft, and the feedlips tend to deform easily under the excessive spring pressure common to all post-ban 10-round magazines. The result is a mag that will sponaneously eject its ammo. These mags should be avoided.

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[4] Drum Magazines
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[4-1] Beta
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The Beta Company manufactures a 100-round, dual-snail drum with replacable feedtowers to fit several types of 5.56mm rifles. These drums are used in some military units in the US and abroad, and are also used by SWAT divisions of several US cities. Most of the pre-ban drums are supplied with AR15-type feed towers, and BATF has ruled that changing the feed tower to a tower for a different type of rifle would be "creating a new hi-cap mag".

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[4-2] MGW
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[4-3] Firepower
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[4-4] Chinese
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