I agree that it appears to be a 37-millimeter caliber projectile. But the photos tell an Artilleryman's eye that it is a Solid-Shot ...a solid (not hollow), non-explosive projectile, a type which is not used against aircraft, but is instead a type of armor-piercing (AP) ammunition. It is generally classified as anti-tank ammo, but of course could be used against other armored targets, such as an armored personnel carrier, or a lightly-armored ship.
Armor-piercing 37mm projectiles have been in widespread use since the very-early 20th century. Unfortunately, this specimen is missing its copper/brass Driving-Band (which also called a sabot). Without it, I cannot say whether this specimen is from the 1930s or 1950s, etc. Nor can I say whether it is American-made or British or French, etc.
I should mention that this projectile's sabot/driving-band being missing indicates the projectile has been fired and was afterward picked up by somebody as a souvenir.
I see that this ID-request is AhhTheHeff's very first post here at TreasureNet, so I'll say "Welcome to the TN forums, AhhTheHeff!"
Let me also say a big THANK YOU to AhhTheHeff, because the many close-up photos (with a ruler in them) show that he (or she) DID bother to read and follow the instructions in the "Please read before posting items in What-Is-It?" message. Many people do not bother to do that -- thereby making the job of What-Is-It forum's ID-helpers much harder. So I'll say again, thank you AhhTheHeff, for the many photos with a ruler right up against the mystery-object.
