As a law abiding citizen you have every right to put the magazine into your pistol.... the law doesn't forbid that. Hence why you can use pistol mags in rifles and enjoy the larger (yet still neutured) capacity of the pistol mags.
The law is written such that it doesn't matter what the mags FIT into, nor does it matter what you PUT them into. It matters what they were designed or manufactured to fit into.
Hence, you could modify your charger stock to accept that drum mag, and it would be totally legal to do so. Hence why people can take an AR mag well for their SKS, VZ, SL8, or many other guns, and use 10 round pistol mags in a rifle, legally.