A pilot would usually only have the target in his sights for a fleeting moment, so a concentration of bullets was vital for achieving a "kill". Even flimsy First World War aircraft often took a surprisingly large number of hits to shoot down, later and larger aircraft were much harder propositions again. There were two obvious solutions—to fit a more efficient gun with a higher cyclic rate of fire, or increase the number of guns carried.A third solution was to replace the rifle calibre weapons with heavy machine guns or cannon: for various reasons this did not become common until the 1940s. Both of these measures impinged on the question of synchronization.
