June 19, 2007: The U.S. Air Force is shifting from high-explosive 20mm shells in its F-22, F-16 and F-15 fighters, because of the risk of those shells exploding in the barrel, and the availability of a 20mm shell that explodes into fragments without the use of explosives. Meanwhile, the U.S. Navy will keep using the explosive round for its F-18s. This is all rather strange. What is going on here?

The current explosive round, the PGU-28, has been in use for about a decade, and has seen heavy use in Iraq and Afghanistan, where improved fire control electronics enable fighters to deliver more precise 20mm cannon attacks on ground targets. This has revealed a minor problem with the PGU-28 round; it sometimes explodes in a hot barrel, that has been firing a lot of rounds. For the air force, this is more than a minor problem, because they mount their 20mm cannon right next to the cockpit. An exploding round can injure the pilot. Not a problem with the F-18, where the cannon is far enough away from the cockpit. The Department of Defense has about eight million PGU-28 rounds in inventory, and is relieved that the navy will continue to use them.

Meanwhile, the air force realized that the German PELE (Penetrator with Enhanced Lateral Effect) round would solve the explosion risk problem. While designed for air-to-air combat, the PELE round is basically a tungsten shell with a plastic filling. When the round hits something, the compression of the plastic causes the tungsten to shatter into fast moving fragments. It's been noted that this happens when it hits the ground, as well as when it hits an aircraft. So the air force is testing it on a variety of ground targets, insure that it has the same anti-personnel effect of the high-explosive PGU-28 round. PELE ammo costs more, but does solve the safety problem of explosive rounds going off in the barrel.