@ingliz firstly, you don't know what the number of small arms deaths was in 1968. You are assuming it was 31.8% of all deaths, combat or not and due to friendly fire or not, as you are assuming the percentage of small arms deaths over the total number of deaths in 1968 was the same as that for the war as a whole. I don't think we know that, but it's important to remember we're making an assumption here.
Secondly, friendly fire deaths count as KIA:
USMC Casualty Procedures Manual, February 1973 via USMC University wrote:Killed in Action (KIA): This term wil be used to describe battle or hostile casualties or those who are killed outright in the presence of the enemy or die of wounds or other injuries before reaching any medical treatment facility. This provides an objective basis for distinction between “Killed Action” and “Died of Wounds” cases as it is often impracticable to determine whether deaths in combat were or were not instantaneous. Personnel mistakenly or accidentally killed by friendly fire in the presence of the enemy and personnel found dead on the battlefield will be considered “Killed in Action.” In the latter cases, the date of death will be determined locally from the available evidence, or if such evidence is not available, the date of death will be the date the remains were found by military personnel. Personnel killed in an aircraft crash en route to or returning from a combat mission will be considered “Killed in Action” provided that the occurrence was directly related to hostile action or the presence of the enemy was a contributing factor. The term “Killed in Action” without the statement “body not recovered” (BNR) indicates the body was recovered.
Thirdly, friendly fire incidents using small arms killed 398 soldiers throughout the war according to subsequent research:
ABC News, 2007 wrote:In previous wars, friendly fire incidents were much more frequent. An Army captain who researched small-arms mishaps in Vietnam found that 398 soldiers were killed due to fratricidal or self-inflicted bloodshed. During World War II, tens of thousands were killed by their fellow soldiers. And by some estimates, 75,000 French soldiers alone were killed by their own troops in the First World War.
As such, it doesn't make much of a difference if they are removed or not. If they are, accidental gun homicides should be removed from the US figures too, which would decrease the total number of gun homicides by around 2% (not a big difference).
Hence the rate of 1,002.4 killed due to small arms fire per 100k soldiers deployed in 1968 is not a completely insane estimate, but relies on the assumption that the share of deaths in 1968 due to small arms fire was the same as the share for the whole war.
And the overall rate of 682 killed by small arms fire per 100k soldiers deployed is not insane either.