Time for another round of what is it....

[Billygoat]

I know exactly what it is. Not firearm related. It is part of an oiler.

Sorry to ruin everybody's fun trying to imagine what diabolical use or weaponry it was part of but it is just part of an oiler.
The dead give away was the top piece with the slot to raise and lower the needle which chokes off the oil drip.

[Billygoat]

I think it probably goes more like this. The tube screws into the base of the oiler with the holes at the bottom to let the oil in. The nut screws down onto the top pressing the top onto the glass. The needle screws into the tube and adjusts the flow. The very top piece with the slot can be flipped down to stop the flow.

The oiler (below) is about 5" diameter x 9" tall overall, is 3/4" NPT and holds a quart. Glass is 1/2" thick. I have some smaller 2" diameter ones on the main journals of my 20" lathe. I probably have 8 or 10 of them laying around in different sizes.

[vegetius]

So, A NON AMMO ITEM!

Smug Mode activated.