i don't think rifle length is the primary concern. that "20 inch aug" still has a 300m range limit, which i'm guessing is due to the cartridge. you could say, well if you made it for 7.62 and gave it a 26" barrel and it was the same length as an m16, then wouldn't that be something? well sure, but again, i think modern 16" ar style rifles with 7.62 have a range out to 800m or so. again, you're just limited by the cartridge. accuracy at that range also is affected by more than barrel length.
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