Canon rounds aren't great for balloon targets. It's quite likely that a balloon doesn't offer enough resistance to trigger the fuzes, which leaves you with a bu

Canon rounds aren't great for balloon targets. It's quite likely that a balloon doesn't offer enough resistance to trigger the fuzes, which leaves you with a bunch of live rounds falling ~40k feet and hitting whatever is below. While self-destructing rounds do exist for the M61 20mm canon, they aren't cleared for aircraft use, only for ground-based use.