Magazines have to be designed to fit both the gun and the bullet being used. If the bullet is too big or too small it won’t go in the magazine. If the magazine doesn’t fit it won’t go in the gun. A fair number of guns share magazines, but I’ve never heard of a magazine that fit multiple bullets.
In this case the gun has been modified to fit a different bullet, which would realistically require replacing pretty much all of the internals. The bit that the magazine fits into is probably just designed to fit whichever magazine is most common.