I know this might cause a bit of debate but I wanted to get some insight before I build AR #2. I already have a mid-length 16" AR, but wanted to build an AR with some optics for some longer distance shooting.
Should I go with an 18" or 20" upper? Does it make a difference?
Thanks.
Barrel quality and optics are going to make a MUCH bigger difference than 2 inches of barrel. There have been confirmed kills in the Middle East out to around 800 yards with the 18 inch barrelled Mk12. Trident82 on this forum has videos of himslef shooting out to 1000 yards with his 18 inch rifle.
Can you elaborate more? Your 16" with a good optic is every bit as capable as an 18" or 20" barrel.
I’m okay with a 16" with good optics. I just want to make sure that it would be an effective platform. Suggestions on optics? I don’t want to kill the bank.
The rule of thumb is that a good scope/optic is going to cost as much as the rifle. I use an ACOG on my SBR at the moment.
On my 16" Recce rifle I was using a Leupold MR/T. Optics are something that you don’t want to buy cheap.
Barrel length in itself is not a determining factor for long range shooting. You have to consider other things along with it: thickness of barrel, quality of manufacture, acceptable weight, etc.
A longer barrel of the same quality will give more velocity, so a slightly flatter trajectory. Not more accurate but flatter. A shorter barrel of the same diameter will be stiffer and probably more accurate, likewise a thicker barrel for a given length. A thicker barrel of the same design will weigh more.
Also, the type of rifling, finishing and tolerances of the bore and rifling affect accuracy and velocity.
I don;t think 2 inches is very material compared to the other factors of diameter, quality of materials and manufacture, etc. If you are shooting long range then a 20" barrel is no less convenient than an 18" one. I suspect you would find a much greater selection of quality barrels in the 20" length.
I’m getting ready to rebuild my 20" AR into an 18" SPR. With the same quality barrel, the 18" will perform just as well as the 20".
I don’t have a ton of experience with long range shooting, but it seems to me the 18" barrel would keep your AR easier to maneuver with keeping it handy.
If your 16" has a decent barrel then invest in really good glass for it and some high quality ammo. If you’re 16" isn’t very accurate I’d go with a 16" medium weight Noveske barrel and something with a Horus reticle, but that’s just me.
I’d grab a Centurion or Noveske 18" for a precision rifle. That’s just me.
16" does pretty good too as stated.
18 is just as good as 20, both use rifle length gas
16 is good too, midlength or intermediate gas length
figure out what what you need to do with it, the only way you would see me running around with a 18 or 20 is if it was some sort of dedicated 3-gun rifle
Actually there’s a few gas system lengths for 18" out there. Just a data point…
yeah i know, theres carbine for 16 too, i was just giving a general overview
Cool cool. I’m just adding to the data I guess. 
I just went through the same process with my new rifle. I ended up with an 18in barrel and for optics a vortex viper pst.
I found no good reason to add the two inches on to the length of the barrel for a few ft per sec difference in velocity.
Here’s my take, and what I discovered, so take it for what it’s worth, I chronographed bot my 18" SPR with a rifle length gas system, and my 20" DMR, both are Douglas SS barrels, after shooting a number of different rounds, I discovered the average feet per second between the two was about 9-10 feet per second off the top of my head. I would have to look it up to be sure, but I do remember I was shocked at how little difference there was, and that number has always stuck in my mind for that reason.
I’m thinking of going the same route, unless I decide to to SBR my recent new stripped lower (Double Star-yes, I know, but it is a lower). If I go the 18" route, what buffer/buffer tube can I use: Carbine or Rifle/A5?
I would go 18". I am looking currently for an upper to fulfill a similar desire and to mate with an A2 lower I have that has a Geiselle National Match trigger installed. 18" allows use of a rifle length gas system, which means smoother and more controllable recoil and followup. 20" offers no appreciable gain in distance or accuracy, just added weight and less maneuverability. There are good reasons why the MK12 SPR uses an 18" barrel and rifle gas system. Just need a quality match-grade barrel and nice low-profile glass.
You can use either just depends on your stock.
So I am by no means an expert in ballistics or barrel design, but I would guess a rifle length barrel length and then loads of training will get you what you want.