These graphs illustrate the diminishing returns of velocity gain as a function of barrel length. We know that for 5.56mm, a 24 inch barrel is a really efficient one. The slope of the velocity curve at 24 inches is equal to the slope of the velocity curve for 6.8 at 19 inches and 300 BLK at 16 inches. 20 inches in 5.56mm matches about 13.5 inches in 300 BLK. 16 inches in 5.56mm is like a 13 inch 6.8 and 10.5 inch 300 BLK in this regard. A 14.5 inch 5.56mm barrel matches a 10 inch 300 BLK barrel as far as slope of velocity gain. An 11 inch 5.56mm barrel is about like a 300 BLK at 7.5 inches. 20 inches in 300 BLK? Might as well be a 29.5 inch 5.56mm barrel in terms of how much left there is to get out of it by going longer - so I would consider that that longest anyone should want to do and almost everyone should stay with 16 inches.
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