Disassembling MP-40 in World of Guns
It's free. It's great a game and learning tool if you're interested enough to see yourself how things (mainly guns) are constructed. World of Guns: Gun Disassembly can be downloaded for free from Steam. Of course a little trick here: it's sort of "pay to play" game where some content can be opened only with your wallet. For example this MP-40 was unlocked after I bought the Guns Lifetime Access. It includes not just WW2 guns, like MP-40, Mauser C96, Luger, Mosin-Nagant but modern weapons as well. Also there's a bonus section where player can disassemble and assemble certain skeletons and vehicles.