I'm looking for long range accuracy for my AR15 lower. What kind of groups can I get and how far can the 5.56 be effective?

And you can't say "get both" because there are three choices.

BTT for the holiday crew

18" is good.  You don't need a lot of barrel length with the 5.56-.223 to be accurate.

I don't think accuracy will vary a whole lot between the three, but you will get more velocity (and thus a longer range) from the 24".

I've got the 20" "varmint" barrel from a few years ago.  (Fluted, free float, etc.)

Works good.

I have a 18 RRA varmint

I bought it mostly because I liked the looks of it better than the other lengths


with gold medal match federal 69gr  it will shoot about 2"-2.5" at 300M if I do my part

even with 5.56mm south african it will shoot just a around 4"at that distance

I have been very happy with the rifle  

I would go with the 20" myself.

Quoted: I'm looking for long range accuracy for my AR15 lower. What kind of groups can I get and how far can the 5.56 be effective? And you can't say "get both" because there are three choices.

I've got a 24" 1:12 twist RRA SS upper on a Bushy lower with an LMT 2-stage trigger.  With the 50 grain Black Hills blue box ammo I can punch sub-MOA groups with it all day long at 100 yards (that's the longest I can shoot on our range).  

The rifle, however, is certainly only good for a bench/prone gun, though... it weighs a ton.  I've tried hiking it around in the woods before and it's too heavy to haul up and down hills, and unless you've got a rest on a tree, sitting on your rear with it resting on your knee, or are prone with it, you can forget about holding it on target easily.  It's just WAY too front-heavy...  It's honestly easier to hold my 16" HBAR with 1 1/2 lbs of suppressor on the end of it while standing than the 24" rifle.

If you're going to be taking it out and hunting with it, then I'd go for an 18" or 20" barrel.  If you're doing bench/prone shooting primarily, then get the 24".  You'll lose a little velocity with the 18" over the 20", but that won't affect your accuracy any.  Accuracy won't necessarily improve with the barrel length, but velocity will go up a little as you go longer, so you'll get a bit more range out of it.

You might have to play with ammo once you get the barrel.  Mine likes what I mentioned above... your might like something different.

If it's gonna be something you're gonna shoot off the bench and not tote around much, I'd go with the 24 just for a little extra velocity downrange.  With a 1:8 twist and a Wylde chamber.

Dito with what Quintin said, the 20 inch with 1:8 twist and don't look back.

20"

In 5.56, anything over 20 is just extra weight.  There is no added benefit, IMHO.