It's a .50 caliber bullet manufactured at the St. Louis Ordnance Plant in 1944. It would have been used for a Browning M2 machine gun, which was first deployed in World War II and has remained in service since. The M2 has been used both as an infantry support weapon as well as being mounted on a wide array of aircraft, ships, and land vehicles for both anti-material and anti-personnel purposes.

If he found it on his farm in this condition it was most likely a souvenir piece that someone dropped shortly before he found it. Modern bullets can last up to ten years when properly stored, but one laying on the ground for years would be far more corroded than this. It may have also been emptied of the old powder, reloaded, and then used by someone who has an M2 as part of their private collection. Your grandfather would likely have heard that though.