The Vietnam sequence is set in 1965, around the time M16 rifles were introduced. Toad and other American soldiers are armed with later M16A1 rifles (distinguishable by their cylindrical muzzle and 30-round magazines) issued in 1967. While the early M16s were indeed notorious for jamming in Viet Nam's in jungle conditions, the M16A1 was specifically introduced to remedy this problem.
Toad is portrayed as a helicopter pilot, wearing the appropriate rank of a warrant officer, yet he is treated as a low-ranking enlisted man who takes orders from the First Sergeant and is placed on details for enlisted men. Normally, this would not be the case, as a warrant officer outranks a First Sergeant, and therefore would not carry out such tasks. Additionally, Toad's poor vision would have most-likely precluded him from being a helicopter pilot in the first place.
The soldier outside the HQ bunker who spots Toad carries and then aims an AK-47 style rifle, supplied to communist forces by the Soviet Union and China. Using enemy weapons would normally be against regulations, although it was not unheard of for US soldiers in Viet Nam to obtain and use weapons like the AK-47. However, this would be very unlikely in relative rear areas like a fire base, especially under a commanding officer like Major Creech.
The final account of the characters states that Terry was reported missing in action in Vietnam. However he staged his death so that his superiors believe he was blown to bits at a specific place and time. Thus he would be reported KIA (killed in action), not missing in action.