' s ▁sub - mun ition , ▁the ▁pipe - b omb - like ▁cyl ind rical ▁M 1 1 4 ▁bomb , ▁was ▁also ▁completed ▁and ▁battle - ready ▁by ▁ 1 9 5 2 . ▁Other ▁delivery ▁systems ▁research ed ▁and ▁at ▁least ▁partially ▁developed ▁during ▁the ▁ 1 9 5 0 s ▁included ▁the ▁E 7 7 ▁bal lo on ▁bomb ▁and ▁the ▁E 8 6 ▁cluster ▁bomb . ▁The ▁peak ▁of ▁U . S . ▁bi ological ▁weapons ▁delivery ▁system ▁development ▁came ▁during ▁the ▁ 1 9 6 0 s . ▁Production ▁of ▁cluster ▁bomb ▁subm un itions ▁began ▁to ▁shift ▁from ▁cyl ind rical ▁to ▁spher ical ▁bom ble ts , ▁which ▁had ▁a ▁larger ▁coverage ▁area . ▁Development ▁of ▁the ▁spher ical ▁E 1 2 0 ▁bom ble t ▁took ▁place ▁in ▁the ▁early ▁ 1 9 6 0 s ▁as ▁did ▁development ▁of ▁the ▁M 1 4 3 ▁bom ble t , ▁similar ▁to ▁the ▁chemical ▁M 1 3 9 ▁bom ble t . ▁The ▁experimental ▁F lett ner ▁rot or ▁bom ble t ▁was ▁also ▁developed ▁during ▁this ▁time ▁period . ▁The ▁F lett ner ▁rot or ▁was ▁called , ▁" pro bably ▁one ▁of ▁the ▁better ▁devices ▁for ▁dis sem in ating ▁micro organ isms ", ▁by ▁William ▁C . ▁Patrick ▁III . ▁ ▁Al leg ed ▁uses ▁According ▁to ▁Le iten berg ▁and ▁Z il in sk as ▁in ▁The ▁Soviet ▁Bi ological ▁We ap ons ▁Program : ▁A ▁History ▁( 2 0