Going by closely studying the photo, the bullet seems to be significantly smaller that the 25mm (a hair less than 1-inch) diameter which you reported. But perhaps as very often happens in close-up photos, "lens perspective distortion" is affecting the view, because the ruler is a good behind the bullet. Please lay the bullet flat, and put the ruler's edge snugly up against the bullet's base... the shoot the photo looking straight down on the bullet (from above).
Yes, it is extremely large for a lead-bodied bullet. Back in the latter-1800s, the US had a 1-inch caliber Gatling machine-gun. See the photo below. But you dug your bullet in the British Virgin Islands, so probably was not fired from a US machine-gun. Then again, some European countries did buy the 1" Gatling from the US. Also, there may have been a British-made counterpart to it... such as the British Nordenfelt Naval Gun. Its bullet is the extremely sharp-pointed one in a photo below. (It was found in the Thames River, at London.) Your BVI-found bullet's undamaged section does closely resemble the British Nordenfelt 1"-caliber lead bullet, in its body-groove depth, and groove spacing. So, that ID would be my guess... if your bullet's base really is 25mm in diameter.
