Ticndig is definitely correct, Keith... it is a finial.
In case you'd like to hear a more-specific ID:
Being found "at the site of an old fortification" (old meaning in this case pre-20th-Century), and being only 1/2-inch long by 3/8th-inch wide, it is most likely a Cap Box finial. That "box" was a small leather pouch which held a soldier's copper percussion-caps, for priming his blackpowder musket or pistol.
When manufactured, your finial had a short round stud projecting out from its base. The stud went through a small hole in the leather cap-box's upper front side, and a rivet-disc was hammered onto the stud to tightly secure it to the box.
Ticndig's photo shows several excavated civil war era cap-box and cartridge-box finials whose riveted base is still intact. A few of those still have a little of the box's leather body preserved between the finial's flat base and the rivet-disc.
The cap-box was worn on the soldier's belt.
I should mention... most of the cap-box and cartridge-box finials we dig are made of brass. The lead (actually, lead-solder or pewter) ones are "said to be" Confederate-made -- but we know some pewter finials were manufactured before the civil war. So, there's no way to know for certain. For example, civilian game-hunters would also use a cap-box... and might have brought it with him when he enlisted into the army.
Almost forgot to mention:
I see your posts in this discussion are your very first posts here at TreasureNet. So, welcome to T-Net's "What Is It?" forum... the very best place on the internet to get unknown objects CORRECTLY identified. I hope you will continue to read this forum -- you can learn a LOT of valuable information for free here. I certainly have. (I hang around to give something back... and, I'm still learning.)
