I'm pretty sure the pin was used as a shooting target. You can see a couple bullet holes in it.
Thanks Medic1210. I went and checked til I found the post by Pieter.45 and you're right there was a mention of shooting pins.
I like to shoot things filled with water or carbonated beverages. I shoot some old electronics with my Ruger Min-14 which was cool but the biggest explosion was an old beat up helmet for driving bikes, cars, go-karts etc.
I filled it with chunks of cinderblock and that little .223 punched through the hlemet and hit the cinderblock inside, effectively blowing it up like it had explosives in it. Great fun!

I'll pose my new XM-18 with some of the carnage next time I shoot.