Want an 18" upper but not really sure what I want.

Want it to run weights from 62 grains and up and be  accurate. Not sure between chrome lined or stainless steel. I hear chrome lined has superior longevity so may go with that if has good accuracy.

The lower is M15 Armalite. I have an ACOG on it and kind of want the ability to use iron sights if the optics fail.  So I have concerns about these flat-top receivers. Although I have never used  the carry handle, a pointless device. My other concern with the carry handle is that the ACOG seats kind of high on it. Maybe get a flat-top and some kind of adapter for it.

LaRue Stealth is always the correct answer.

ETA: Chrome lining reduces accuracy and you don't want to mount an optic on top of a carry handle.  Use a quality mount on a flat top.

The stealth is nice, but so is the BCM SS410 upper and it's a little easier on the pockets.

You could go with a 6.5 Grendel upper in 16" and have a longer range to boot. At least that's what I ended up doing.

Seriously though, you would be good with anything from BCM or LaRue, just make sure that you pick a stainless barrel instead of chrome-moly with chrome lining.

SS = more accuracy I presume?

I recently built an upper with a Rainier 18" Ultramatch and it shoots much better than I do.   Take a look at the Rainier Ultramatch uppers.

As an aside,  I haven't had good results with 62 grn in any of my rifles... you might want to look at some of the ammo reviews by Molon or Cody47.   I get very good results with 69 grn, 75 grn and just about any ballistic tip ammo.

When you start talking about accuracy builds, you will get to a point where just about everything in the class is going to be good and will be more of a matter of whether you and your ammunition are up to the task.  I haven't tried everything, but recently had a BCM SS410 16" build that consistently shot sub MOA.  I got bored with it, took it apart and sold the barrel.  I then sold the rifle.  Then I built another using a Superior Barrels 18" barrel.  Took it out on it's maiden voyage and it also did consistent sub MOA with my favorite accuracy load.  I'm sure the Ranier Ultramatch Shillen barrels or the Larue or the Lothar Walther barrels from Spike's will all do the same.

If you are buying something that's already built, just buy from a dealer that will stand behind the product and practice a lot. I would, however, suggest that you opt for the rifle length gas system.  Smooth shootin.

There are a number of very reliable and accurate 18" upper available. Of course I could tout my own, but the LaRue, Noveske, and BCM are all very accurate and durable guns. If your looking for accuracy go 410 SS or a nitrided CM. Twist rates for the bullets would be 1/7, 1/7.7 (Kreiger), or 1/8. Some folks report a bit of gyro instability under 100 yards with 1/7 and heavier bullets.

I've had great results at 100 with my 16" 1/7.7 Kreiger and 77gr ammo, handloads or factory match.  No "gyro" instability there apparently.

Besides how would you be able to tell if a bullet was "wobbling" through the air?  If it keyholes in your paper it's doing a lot more than wobbling.  It's either stable or not... the "gyro" or yaw rate of any projectile becomes apparent if there is not enough spin or if the projectile has a flaw and is somehow out of balance. (Just like some green tip M855 penetrators)

The more practical thing to assume is that as you increase bullet weight, recoil impulse also increases (its a slight increase but it is noticable) therefore a shooters follow through or inconsistency with follow through will become more apparent.  In the AR, as I have said before, follow through not only with the trigger but the entire position and hold is essential to being consistent.  That is also why some shooters lead-fill the A2 stocks to add weight.  A heavier gun will generally "absorb" the recoil impulses better than a lighter gun which in turn translates to less movement of the rifle and that makes it "easier" to follow through consistently.