Cyclic round

I have had this for ages I did not paint it this is how it turned up along with a WWII Bofor 40mm in the same paint scheme. I do not know what cal it is only the owner said it was to check the cyclic rates of a gun (GAU!) I am trying to find out a bit more on it the Boyes .55 inch is for comparison, the projectile has no markings, the screw that goes in the primer hole has SPS on it, the base has 087 M 96 (DOM?) as always I appreciate members input TIA

Some of these bigger guns have a number of solid rounds in the feed to engage before the live rounds come through. I've always wanted some, I was told the A10 used them. I've also seen them for the Bushmaster. In that case though, I'm wondering if they might just be drill rounds to test function. Same as we had for all the others, .50 and down?