I think you need to clarify the type of magazine you're using, be it real steel, or airsoft, and the type of gun it will be used with.
In general, it's difficult to modify an airsoft gun magazine to be "longer or shorter" than the way it's manufactured. Airsoft mags in general have plastic internal components which are placed in the magazine to function in a certain way.
Now, if your idea is to make a large capacity magazine, like something up in the 1,000-4,000 round range, that IS doable provided you limit any mods to the mag to just the metal casing itself, and not the internal structure.
For instance, if you wanted to make, say a 1,000 round G3 mag, you'd need to make a box that the mag would be mounted into, and from there you could cut a hole in the side of the mag to allow an insane amount of BBs to feed into the gearworks of the mag.
Essentially what you'd be doing is enlarging the "BB well" inside the magazine, with an external addition. You could also modify it even further by adding an electric gear winding motor and slave that to a push button contact switch.
That's what I believe Guarder did with the plans for their M249 magazine. They decided not to produce the mag itself, and posted the plans for anyone to use.
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