What made this hole in my helmet?

[Thanatos]

Guys this lid could have just been lying on the ground when damaged , helmets were lost on the battlefield for many reasons and subsequently damaged by small arms , schrapnel , target practice/zeroing etc etc so it's impossible to draw any conclusions as to where it was damaged !!

yep 110% correct.

[DougB]

I don't think it was damaged by ordnance of any type. The folded edge of the rim is not damaged from an impact and the hole is too perfect. A large nail or spike was what did this I believe, the damage is centered on the edge of the skirt, the perfect place to pin a helmet above a barn door for example. I've seen a few helmets with holes in similar ways and they were all pinned up on a wall of some sort.

[brataccas]

maybe it was used as a flowerpot or a waterbucket

[Pablo]

I assume the liner is missing........if it was still there you can easily check for blood residue with a luminol test.