What is this small calibre cannon round?

I found this on UK ebay:

It appears to be about 15mm Calibre, with a rebated case and a bullet with copper driving band. I have never seen anything like it before. It looks to have benn nickeled by a third party at a later date.


It looks bigger than 15mm. Compare it with the size of the '06 dummies.

It could be just about anything. Case made from an unknown shell? A 20mm case necked down? A completely new case lathe turned? The “projectile” could be fabricated from a piece of steel or brass. The smaller one in his store looks like it was made from a 50 BMG.

I have one that is very similar to it made from a 20mm case with the “projectile” being made from brass stock and drilled to hold the lighter.

Both of these look like they were made commercially and not by some GI as he implies.


Looks to me like the one on the far right of this pic (from the Ammunition Photo Gallery on my website), and the overall length is correct:

It is the US experimental 20x158RB T5, for the post-WW2 T33 aircraft gun.

That’s quite a rare beast, well worth having. And no bids yet? Go for it!

It sure looks the the 20 x 158RB T33. I may well go for it. Could the nickel plating be easily removed? I think I would take it off the base, remove the plating, and remove the lighter mechanism if it is a rare round. I also get the four fluted '06 dummies which I don’t have. Its only