▁loaded ▁four - to - one , ▁on ▁both ▁fixed ▁off ensive ▁and ▁flexible ▁def ensive ▁guns , ▁to ▁help ▁mit ig ate ▁the ▁difficulties ▁of ▁aer ial ▁g unn ery . ▁T rac ers ▁were ▁very ▁common ▁on ▁most ▁W W II ▁aircraft , ▁with ▁the ▁exception ▁of ▁night ▁f igh ters , ▁which ▁needed ▁to ▁be ▁able ▁to ▁attack ▁and ▁shoot ▁down ▁the ▁enemy ▁before ▁they ▁realized ▁they ▁were ▁under ▁attack , ▁and ▁without ▁bet ray ing ▁their ▁own ▁location ▁to ▁the ▁enemy ▁def ensive ▁g unn ers . ▁The ▁United ▁States ▁re lied ▁heavily ▁on ▁trac er ▁am mun ition ▁for ▁the ▁def ensive ▁Brow ning ▁M 2 ▁. 5 0 ▁cal iber ▁machine ▁guns ▁on ▁its ▁heavy ▁bom bers ▁such ▁as ▁the ▁B - 2 4 ▁Liber ator , ▁but ▁they ▁found ▁that ▁during ▁def lection ▁shooting , ▁g unn ers ▁who ▁tried ▁to ▁aim ▁using ▁the ▁trac ers ▁rather ▁than ▁the ▁s ights ▁ended ▁up ▁not ▁" leading " ▁the ▁enemy ▁aircraft ▁enough ▁and ▁missing , ▁because ▁of ▁an ▁optical ▁ill usion ▁that ▁made ▁it ▁appear ▁that ▁the ▁trac ers ▁were ▁striking ▁the ▁aircraft , ▁when ▁in ▁reality ▁they ▁were ▁passing ▁significantly ▁behind ▁it . ▁This ▁proved ▁true ▁of ▁fixed ▁forward ▁firing ▁guns ▁as ▁well . ▁In ▁some ▁cases , ▁g unn ers ▁were ▁ordered ▁not ▁to ▁use ▁trac er ▁am mun ition ▁at ▁all , ▁but ▁in ▁the ▁end , ▁with ▁greater ▁emphas is ▁during ▁training ▁on ▁using ▁the ▁s ights ▁and ▁not ▁trust ing ▁the ▁trac