I have gone back and forth between red dots, red dots w/ magnifiers, and variable optics on my SBR.
I use a 12.5" barrel for preference, and my current BCM barrel is capable of 1.5 MOA or so on any given day with good ammunition. This can easily stretch its legs to 400m if I needed it to (farther if it isn't too windy), so a low powered, variable optic makes a certain amount of sense. My favorite for this task is the Leupold Mark6 1-6x20mm. I have used this scope on 10.5", 12.5", 14.5", and (currently on) 18" ARs in 5.56 and SCAR-17 in .308, and it is frankly the perfect optic for what and how I shoot, pretty much regardless of platform. I've used it to hit steel at 1050yds on my SCAR.
Glass is fantastic. With the TMR reticle, it isn't beholden to one cartridge or barrel length, provided you do your homework and learn your hold overs. Caliber-specific BDC reticles are available, if you prefer them. It is lighter than even many 1-4x optics on the market and has very positive adjustments should you need to dial. The downside is that the reticle illumination is about 75% as bright as I would like it to be. Daylight visible, yes, but not super bright if there is, say, snow on the ground reflecting a ton of sunlight. This is something they really need to address.
TL/DR: The Mk6 fits the bill for me if I ever want magnification on my 12.5" gun. I would recommend it to anyone and I will be buying another.