Hard to be conclusive, but in general "not much" appears to be the answer; I haven't come across any accounts of casualties from falling machine gun bullets at least. Why Don't Penguins' Feet Freeze?, a collection of questions and answers from the 'Last Word' column of the New Scientist, has a question on the potential lethality of bullets fired into the air during celebrations (a question that's also come up on e.g. [Slate] (http://www.slate.com/articles/news_and_politics/explainer/2011/03/watch_out_for_falling_bullets.html) and [Mythbusters] (https://mythresults.com/episode50)). One of the answers was from M. W. Evans who collected spent cartridge cases during the Battle of Britain that "drifted down slowly from the sky" and suggested that a small projectile like a .303 bullet would do "nobody much harm when it lands".

More of a threat than bullets were shells and shrapnel from anti-aircraft guns falling back to earth, these certainly caused casualties but it's difficult to assemble firm data. See posts from myself and /u/Georgy_K_Zhukov in [this thread from a while back] (https://www.reddit.com/r/AskHistorians/comments/6gyj6f/were_the_effects_of_wwii_aaflak_rounds_falling/diu81va/) for a couple of examples.