If you want to see beautiful color photos of the MP18/I being shot and field stripped, as well as the story of the gun, including what it feels like to shoot it, see Robert Bruce, "Machine Guns of World War I", pp. 46-57. Whole book highly recommended, also "German Automatic Weapons of WWII," same author. Wonderful books! Timothy Mullin, "The Fighting Submachine Gun...", has a review and shooting impressions of the gun, H.D. Gotz, "German Military Rifles and Machine Pistols, 1871-1945", has nice exploded views of the snail drum magazine, and of course, Thos. Nelson, "The World's Submachine Guns" has excellent coverage of this weapon. There is even more available on the MP28II, which was the same gun modified to take stick magazines.