Rings made from spent bullet casings by Brendon Jaine in his backyard workshop in Invercargill, New Zealand. 

Material: Brass, spent 308 bullet casing

Ring size Aus/NZ: U - Z Adjustable

Interior Diameter: ~22mm

Would fit: large man's finger

Measurements are approximate because these rings are not perfectly round, so sizes are guesstimates/averages. The bullet rings are all easily adjustable. 

They have some pitting and dints from their previous use, and all are slightly different from each other, and may have slight differences from these in the photos.