Eventually, German pilots developed new combat tactics to counter Allied bombers' defences. Me 262s, equipped with up to 24 unguided folding-fin R4M rockets—12 in each of two underwing racks, outboard of the engine nacelle—approached from the side of a bomber formation, where their silhouettes were widest, and while still out of range of the bombers' machine guns, fired a salvo of rockets with strongly brisant Hexogen-filled warheads, exactly the same explosive in the shells fired by the Me 262A's quartet of MK 108 cannon. One or two of these rockets could down even the famously rugged Boeing B-17 Flying Fortress, from the "metal-shattering" brisant effect of the fast-flying rocket's explosive warhead. The much more massive BR 21 large-calibre rockets, used from their tubular launchers in undernose locations for an Me 262A's use (one either side of the nosewheel well) were only as fast as the MK 108's shells.