Because I honestly don't know the answer, but has the MUR shown a quantifiable improvement in accuracy over a standard upper receiver when used?
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If so, I have never seen it
Your barrel will probably play more into the accuracy of your build more than upper receiver, as long as it's in spec. Aero will work well, but Vltor/Geissele is a step or two above, in both price and build quality.
If you'd like to save a little, BCM uppers are also built a little thicker in areas and have VERY tight receiver to barrel tolerance.
My MUR with Geissele rail and LaRue barrel shoots sub-MOA as long as I do my part and feed it quality food.
Okay, perhaps I embellished lightly. 8" at 800 yards is beyond my shooting capability. It'll do approximately 1" groups at a 100yds until I flinch (and it is capable of .75" at that distance with BH ammo) and it'll ring steel to 600 all day which is as far as I've verified it.
I have no evidence it is more capable than any of my other rifles, but I can say it is capable for longer simply due to the barrel's thermal mass. I have not noticed deflection of the handguard either, which is present in some of weight weenie guns.
If you don't want to go monolithic, I'd say another option with the same benefits (but easier to work on) is the Mega MML (M-Lok) or MKM (Keymod). Here's a review (not mine) that illustrates the features pretty well:
https://youtu.be/YcjhEjeDNLk
I have four of these and they are all outstanding.
I don't own a VLTOR but I have been impressed with the M4e1.
My daughter who was 15 at the time shot this 10 shot group @ 100m with an M4e1 16" govt profile bbl using some M855 and an 8x vortex scope.
http://i288.photobucket.com/albums/l...pspi0xisq7.jpg
http://i288.photobucket.com/albums/l...psifeoygog.jpg
I was impressed enough to recently purchase a 18" stainless medium profile 1/8 .223 Wylde M4e1 upper. I added a Vortex Viper 2.5-10x44.
http://i288.photobucket.com/albums/l...pszu3n0lnk.jpg
http://i288.photobucket.com/albums/l...ps7hab2lin.jpg
...and you handload. For that type of split ammo is going to be the deciding factor, not the upper. That being said, the biggest argument against the MUR is the forward assist. Its design prevents the use of a lot of 45 offset iron options, which kind of makes it a non-starter for me.