Actually, they tried the weapon without it. Early M16s (USAF, Project AGILE stuff from '59-'63) lacked the forward assist. Some of these weapons were used by US advisors in Vietnam. When a need for a way to manually close the bolt was discovered, the forward assist was added starting with model 604. Stoner didn't go to war with the weapon; while he came up with a unique operating system, he's probably not the best source of information on what other features are necessary on a fighting carbine.