Everybody at Robb's place the other day was swooning over the GAU-8 Avenger, the famous 30mm tank-busting cannon around which they built A-10 Warthogs. It fires a projectile weighing just less than a pound at some 3,500fps, giving it a muzzle energy of 178,427 ft/lbs. Also, its high rate of fire corrects the biggest shortcoming of the previous super-heavy U.S. aircraft champ (forward-firing division*), the 75mm T13 on the B-25 Mitchell, which chugged out its 16lb/2300fps shells so slowly that only a couple could be gotten off on a single strafing run.
Of course, compared to the Army's 120mm M256, flying cannons are mere popguns. The depleted uranium long-rod penetrator from the main gun of an Abrams weighs the next thing to 18 pounds and is tooling along at 5500 ft/sec or so.
Let's see... 18 pounds is... 126,000 grains... at fifty-five-hundred feet a second, that comes to... umm... carry the one...
About 8,465,485 foot pounds of muzzle energy, which is definitely going to make Major.
All of these, though, are small potatoes when compared to the now-sadly-extinct Mark 7 16"/50 caliber Naval Rifle, which accelerated a 2,700lb projectile to twenty five hundred feet a second in the length of its 66-foot-long barrel. (What's a minute of angle at 32,000 yards?)