Have a few SbR‘d lowers, and my answer is ‘it depends’ … what do you want to do with this rifle? I have multiple uppers for each lower for slightly different needs.
For example, this one’s a 300 BLK upper in a ‘fits in a tennis racket bag’ PDW form, but no problems taking the MRO out to 100 yards. I do a 50-yd zero for supers, and 25-yd zero for subs. This one is fed almost 100% subsonic ammo.
One of my favorites is this 5.56 with an ACOG and stacked RMR; perfect for heads up shooting on NVGs. Once I shoot this barrel out, I’ll also swap the hand guard for something with a rail and get a nice IR laser to go with this optic duo. But I just love shooting this one like it is for now…
…also have a 300BLK upper for that one; but with a (now) old school Leupy MK4 1.5-5 FFP with the 300BLK reticle. May swap scope mounts and add a offset red dot to this one…have grown fond of that setup with an LPVO:
Edit: meant add this 12.5” 5.56 upper in as an example…rapidly becoming my favorite AR combo (LPVO plus offset red dot or offset iron sights)… Heavy as hell though, especially with S&B glass, LoL
Here’s another 300BLK upper with a red dot and magnifier on a flip up qd mount…again, horses for courses, and this one is almost identical to another one in 5.56 sitting in a safe at my son’s house; 10.5” barrel on the 5.56 upper vs. 8.5” here though:
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…have a few others, but they’re basically variants on these themes; my son likes Eotech red dots, well, holographic sights to be precise, but they look ‘fuzzy’ to me, so I typically use Aimpoint or Trijicon…or cheap Primary Arms micro dots like posted above. They work great for the $$, but is it sacrilege to pair a PA red dot with an Eotech magnifier and Unity mounts? LoL
Bottom line: For an SBR, I like red dots, with or without a magnifier, or LPVOs paired with some form of offset red dot or irons for close up work….YMMV.