For some reason gun bbls in the AR family are either < 16" or >20".
I can tell you in the jungles of Vietnam, the Piney woods of E Texas, the Mesquite of S Tex, the Central highlands of Ca from San Fran S to just N of LA, Ga, FL, Va, Md, Ks, Ak, Hi, Wa, Korea, Ger, most of Europe except the Med, Latin America, S America the carbine is your friend. Head to the ME across the 'stan' countries, Israel, N Africa, most of Aus its rifle time.
Just try to remove a water pump on a Chevy V8 using a pipe wrench...right tool for the job at hand.
This takes me to my opening statement. I was wanting to do a build, a high end build top end components. Almost every bbl maker I went to would not build a 18" bbl, always saying its less accurate than a 20"..yes it is.
So time for some research: It was not easy but I was able to pull up data on bbl lengths from 10.5 all the way out to 26". Not all was using the same ammo or loads or even the same gun other than AR format. Here is what I found:
fps and accuracy and other measured data. The difference between bbl at about 2" intervals: 10.5, 12, 14, 16, 20, 22, 24, 26 (could not find enough data on 18" bbl). The difference in bbl length is fairly linear across the lengths. example only: 26" = 3200 fps; 24" = 3000 fps, etc. Point: the measured data in bbl lengths is not on the order of magnitude unless you take the difference between a 26 and a 10.5 (for example), the diff between 26 and 24 is minor, but remember it does add up!
All this begs the question: WHAT is the accuracy REQUIREMENT? I PERSONALLY say its 'pie plate'. Standard pie plate is 9 inches. Size of a humans head, chest shot, Deer, Elk, Bear, Rino and we can go on and on. Get it in the pie plate in a critical zone and its going down!
That said this is not going to be a poster child for the Target and Sniper set.
Then for most us, especially if you are a soldier or hunter, pie plate accuracy works for you. Then the only question is typical engagement range of your area of operations? For most of the AO's in the aforementioned sentence it would be between 50 and 150 yds. The other place such as the ME, its 150 ++++ or "Designated Marksman" space.
Finally, using this data and posing the question carbine <16" or rifle >20" is there not a compromise at 18"?????? IF so then why is the 18" bbl length almost void?
