While there is a plethora of companies offering to do modifications to the base firearm, manufacture accessories, or do various custom machining work, few focus on the magazine of a given firearms. Outside of the option to develop and produce a new magazine, few spend the time to modify existing designs.
To fill the niche is a small company named Bad Element Company, which offers custom magazine services. Their work spans across a variety of platforms including M14/M1A, AK, AR-15, and others that may blend the platforms such as a posted option to convert M14 magazines to use in a Zastava PAP M77 .308 AK.
Other specialties include converting full-capacity options into ban-state compliant models such as taking 30 round STANAG or other magazines and riveting them to never hold more than the locality requires. Taking it a step further (or potentially taking the modification to an absurd level), Bad Element will convert 75 round drum magazines into 10 round compliant models.
For those looking for something more usable, Bad Element can “cut down” the magazine to a shorter length. The most popular option includes cutting down bakelite, plum, polymer, or steel AK magazines into the “tanker” configuration.
On the unique side, Bad Element also offers custom magazine windows. While utterly impractical (yes, one can see their round count), it is beautiful.
Those interested can check out his website here. Prices, services, and other items are detailed.