I want to build a AR-15 in 5.56 and I'm interested in what length of barrel I should get. I'm not in a state where SBRs are legal. I have a 300 BLK pistol with a 10" barrel that I intend to use for home defense. So the AR-15 rifle would be for long range shooting, shooting at the range with less expensive .223 ordnance and SHTF.

I don't want to get a 14.5" barrel with permanently attached muzzle device because I would want to be able to switch out the gas tube if I needed to, or replace the muzzle device. So my debate is between the 16", 18" and 20" barrel lengths. I like the idea of the 20" barrel because the 5.56 was original designed for this length, and you get optimal velocity and full powder burn. My concern is that it would be too heavy and unwieldy.

My first build has a 20" barrel, rifle gas and a bayonet lug.  Good looking, ultra reliable and quite handy.  It turned out to be my most accurate rifle, with its Green Mountain 1in8 barrel.  Weight is 7.5 lbs with iron sights.  I find it ironic that what we think of as a "full size rifle" today is actually shorter and lighter than the carbines carried by the horse cavalry.  - CW

http://www.gmriflebarrel.com/gm-m16-m16a2-5-56mm-20-barrel-with-extension-1-8-twist/

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I tend to favor 20” barrels, but I think 18” barrels are a good compromise for SHTF and longer range shooting.
How heavy or unwieldy a 20” will seem probably has more to do with physical condition and size of the shooter.

For an A4 style rifle with carry handle and a fixed stock you're looking at ~39.5 inches in length and 7.5-7.7 lbs unloaded.

It's still lightweight and compact...