Ah, Reddit. The perfect embodiment of all that is wonderful, and monstrous, about the web.
Yesterday a team of historians worked together to produce this excellent discussion of small arms in the World War One era. It's a great introduction to the subject filled with links to more in-depth resources and further reading. I'd recommend it to anyone with the slightest interest in what kind of weapons pop up during classic-era Mythos adventures.
As a bonus, you won't have to wait for Gawker, Buzzfeed, or the Huffington Post to recycle it into some inane "You Won't Believe These Ten Insane Weapons of World War One" click-bait.