Going long in 5.56?

[FireMedic]

I'm thinking of building a DMR style 5.56 AR platform. Currently my only 5.56 is a 16" andI am on the fence about either going to 18" or all the way to 20". This would be more of a range gun than anything practical/hunting (but who knows) so weight and length is not an issue at the moment. I have done some research on muzzle velocity and the 20 is superior. Additionally, I know the good lord Eugene designed the 5.56 to be fired from a 20" barrel. However, it is much easier to find manufacturers with inventory in the 18" barrels than 20" barrels. Which leads me to wonder if there is an appreciable difference between the two for a low-no speed floperator. My usual go to manufacture is Aero Precision and they do carry the 20". I just wanted to pick the mind of more educated/informed shooters before I blindly commit to 20" just because that's the way of Stoner.  

[Whisper]

Interesting question. I've shot 1000 yards a few times with an HBAR but have no experience with an 18-in barrel and I'll be watching this thread to see if anyone has experience to share.

[gregintenn]

Way back when the TGO receivers were offered here, I built a rifle on a set with a 20" barrel from Yankee Hill. This is as accurate a rifle as I've ever had. I know that is only an experience of one, but I would recommend a look just because of it.

https://yhm.net/accessories/barrels/20-fluted-melonite-qpq-barrels-1/

Kinda like this, but i'm thinking mine is a 1 in 8 twist.

[Alleycat72]

My go to ar 15 is an 18". It's relatively easy to keep rounds on a human silhouette at 750-850. After that things get bad. I hold a 10" group at 750 with that gun if I want to be honest.  At 1000, it twice that.

If it's just a range toy without a suppressor, I'd do a 24. A 20" will get it done if you math the s÷$t out of the bullet. An 18" will if you get lucky.

My 18" is really only for killing, so take it for what it is, but I did quite a bit of long range testing before it was relegated to this role.

Fast twist,  heavy for caliber, and big BC numbers are what you're after. 

[Chemgeo]

If you're going for a true DMR and not an SPR or RECCE setup I would suggest a larger round. 5.56 is just not a 1000 yard cartridge. The terminal ballistics will be terrible at that range. If you have even a little bit of wind you're going to be absolutely all over the place at that distance. 750 is doable but still pretty tough, whereas with something like 6.5 creedmoor it's almost boringly easy. So with that in mind you're probably building a 500-600 yard gun, which is still really fun and quite a long shot and neat to have. 

As far as barrel length, honestly 16" is adequate to shoot that distance, but obviously 18 or 20 is a bit better. Between 18 and 20 it's really not a noticeable difference or something you would need to be concerned about. If you want a recommendation, Ranier Arms ultramatch barrels are excellent and reasonably priced and come in several lengths. Larue also produces very accurate barrels.

The real decision you're going to have to make is optics. A lot of guys put way too much magnification and objective size on a gun that is 1) supposed to be reasonably light and mobile in the case of a RECCE or SPR, and 2) isn't going to shoot past 600 yards. 2-10x, 3-14x, maybe 4-16 x40 ish are all reasonable magnification rages that keep weight down and provide target ID at range. 

scopes that are light, clear glass, turrets that track well, and durable, are going to be expensive though. 

[NGraham]

20 inches turns target ammo into battle ammo

[Frog4aday]

I vote getting the 20" barrel. Stoner had it right at inception. Rifle length gas system is reliable & gentle on the bolt & brass and the velocity gain is a bonus. Might even be a tad quieter, but not enough so to put a big checkmark by that in the "pro" column.

[DL126]

I suppose interpretations may differ, but read the linked article.
If it's accurate, my interpretation is that Mr. Stoner never intended for the AR to be used the way we use them today.
Modern day rifles have morphed quite a lot from the original design.
Most specifically in twist rate, which has a great impact on usage scenarios.

Published in 1981

ETA:
In response to OP ....
I have an 18" SPR in .223 Wylde that is a great shooter and much enjoyment.
Obviously it'll do stuff out to 600 or better.
But I have built 6.5 Grendels and a 6MM ARC for the stuff beyond 450-500.
I stay in small frame calibers simply for the sake of parts commonality.

Edited July 15, 2022 by DL126

[Beltfed]

If you wish to hit something at 1,000 yards, get a 60mm mortar.  Being issued a M16 after training on a M14, anything beyond 200 yards, you’ve are just wasting ammo.  Bring up the ‘pig’ (M60).

Edited July 15, 2022 by Beltfed

[aknifemaker]

Check out White Oak Armament, a lot of stuff for the long range competitive shooter.