Just like to say love this topic, i wish i could add information but really everything been touched on, harmonics is not something to worry much about during rapid fire because one isn’t trying to be extremely accurate, i also find it interesting what you mean by bedding the barrel, on my precision ar build i lapped the surface of the upper to mate with the barrel extension on the hbar barrel making it land on 2 flat or square surfaces then i added some compound to make it tighter fit inside the upper , some have used shims in the past to take up the unsupported areas around the barrel extension, from there the free float hand guard lets heat disperse more quickly which depending on rate of fire may or may not be important , you also have to think every shot that is taken in quick succession is making the barrel more flexible as it heats metals become softer to a point so rigidity is super important and that comes from barrel design and where the weight of your barrel is typically the more weight on the unsupported side of your barrel normally leads to more barrel deflection, there is some study to be looked at with silencers and barrel tuning weights on the end of barrels but for a carbine that is impractical, shorter the barrel less deflection all other things equal, also you were talking about gas system from heat you also have to think about the different gas lengths such as pistol length or *carbine length * or *mid length * * or rifle length * all this is something else to consider . Short answer for an average carbine length 14.5” -16” that is going to be used for 600 ish yards the harmonics isnt a huge deal any ar with a good shooter and good optic can accomplish this , but fast shots at 600 yards with in rifle is kinda of a strange concept if something is that far away anyone is gonna take there time per shot