<Li> A detachable box magazine is a self - contained mechanism capable of being loaded or unloaded while detached from the host firearm . They are attached via a slot in the firearm receiver, usually below the action but occasionally to the side (Sten, FG 42, Johnson LMG) or on top (Madsen machine gun, Bren gun, FN P90). When necessary, the magazine can easily be detached from the firearm and replaced by another . This significantly speeds the process of reloading, allowing the operator quick access to ammunition . This type of magazine may be straight or curved, the curve being necessary if the rifle uses rimmed ammunition or ammunition with a tapered case . Detachable box magazines may be metal or plastic . The plastic magazines are sometimes partially transparent so the operator can easily check the remaining ammunition . Box magazines are often affixed to each other with clamps, clips, tape, straps, or built - in studs to facilitate faster reloading: see jungle style . </Li> <P> There are, however, exceptions to these rules . The Lee--Enfield rifle had a detachable box magazine only to facilitate cleaning . The Lee--Enfield magazine did open, permitting rapid unloading of the magazine without having to operate the bolt - action repeatedly to unload the magazine . Others, like the Breda Modello 30, had a fixed protruding magazine that resembled a conventional detachable box but was non-detachable . </P> <P> The FN P90 personal defense weapon uses a horizontally mounted feeding system; the magazine sits parallel to the barrel, fitting flush with the top of the receiver, and the ammunition is rotated 90 degrees by a spiral feed ramp before being chambered . The AR - 57, also known as the AR Five - seven, is an upper receiver for the AR - 15 / M16 rifle lower receiver, firing FN 5.7 × 28mm rounds from standard FN P90 magazines . </P> <P> Another form of box magazine, sometimes referred to as a "quad - column", can hold a great amount of ammunition . It is wider than a standard magazine, but retains the same length . Casket magazines can be found on the Suomi KP / - 31, Hafdasa C - 4, Spectre M4, QCW - 05 and on 5.45 × 39mm AK rifle derivatives . Magpul has been granted a patent for a STANAG compatible casket magazine, and such a magazine was also debuted by SureFire in December 2010, and is now sold as the MAG5 - 60 and MAG5 - 100 High Capacity Magazine (HCM) in 60 - and 100 - round capacities, respectively, in 5.56 mm for AR - 15 compatible with M4 / M16 / AR - 15 variants and other firearms that accept STANAG 4179 magazines . Izhmash has also developed a casket magazine for the AK - 12 . </P>

How to tell how many bullets in magazine