Location: Sydney, Australia. Found it during bushcare.
The brass barb fitting and the powdery filling suggest some sort of kiln burner to me, but the dark green paint on the outside of the tube looks rather ordinary and not like it has been through high temperatures.
The soft, powdery cemetitious filling has a copper-green tint. Only one end has a hole.
If it were not for the brass barb and coppery fill colour I would assume this is just a bit of structural steel from someone's carport (or similar) that has filled with cement and now been cut to pieces for disposal. But a carport with a barb fitting? WTH?
We find all sorts of garbage in this bushland because it's sandwiched in suburbia. Traditionally it was a dumping ground (mattresses, furniture, asbestos, whole cars) and today still people use it illegally as a dump (mainly building materials and soil). Lots of random materials get deposited by or uncovered by stormwater runoff & floods too. There is no limit to the craziness of what you find here.
I've built plink-around cannons like this - fuse goes through the hose barb, cement inside, cast around a dowel and bam, cannon. Not saying that's what this absolutely is, but it's yet another option.
Hmm. Sounds like fun lol