Are the Finnish and Soviet drum magazines interchangeable? Never had both to compare. Friend showed me a semi-auto he had blank adapted for reenacting the Winter War. Any excuse to buy a new gun.

You would think so, b/c the early Soviet PPSh-41s actually took the 71 rd drum mag from the Suomi. Seems probable and yet still questionable as the Suomi was more versatile and could take 4 box mags (20, 36, 40, and 50 rd) + 2 drums (50 and 71 rd). The PPSh-41 by compariosn took only 1 mag (the 35 rd) and the copied 71 rd Suomi type. The PPS-43 took no drum, just a 35 rd box mag. Throughout weapon history, adaptations for enemy weapons and ammunition have been made and direct copying. But even if the later, any slight change can affect the compatibility. Maybe Susanna Viljanen might know! The Germans modified many foreign arms captured to take 9mm Parabellum in one form or another. Interesting story from WW2: The Germans copied the Sten Mk.2 as Gerat Potsdam and manufactured nearly 30,000 for guerilla warfare. The copies were so exact that Brit markings were copied as well. But they sat in stockpiles, not intended for the German Army at all. A few are seen with Waffen SS as propaganda pics, but it never saw any real action. A complete waste of time on another exotic scheme. The Werwolves that did fight post May-1945 used German Army, foreign, and improvised weapons (such as cane and umbrella guns). No Gerat Potsdam, EMP 44, nor MP-3008!!!

I once saw one of those Volksgewehr 7.92x33mm. Flat board stock. Magazine from an MP-44. Very crude, but I heard they worked. Brought a lot of money at auction.

Patrick, the VSG was a Gustloff 1-5 series and was used in 1945 by the Volkssturm and some HJ/Werwolf guerillas postwar. Gustloff had tried to sell the German Polizei its P.40 of 7.65mm in 1940 and failed, even after sending one to Hitler himself. I find that the Volkspistolen were more interesting and here is a Russian page with most of them:
http://xn--80aaxgqbdi.xn--p1ai/publ/8-1-0-171