I'm either going to purchase or build a lightweight 16" carbine in .223/5.56.  I purchased a White Oak Armament 16" SPR upper thinking it was going to be what I was looking for, but it feels a bit heavier than I'm looking for.
I'm now looking into Faxon, Daniel Defense, and other manufacturers for components or a ready-to-go upper.
So, buy or build?

Really depends on your goals, but I’m a bit confused on how your search for a LW upper led you to a WOA SPR profile barrel.  Those things aren’t even in the right galaxy as far as “lightweight” goes.  I know, I had that exact barrel on my nighttime yote gun for a few years and it was carried afield a lot.  I swapped to a Proof CF for a reason.

As for buying or building, I’d identify if you have any specific parts you’re wanting (muzzle device, handguard, receiver, BCG, etc) and go from there.  If someone manufactures an upper that checks all your boxes, then roll with that.  If not, look into going the build route.

I’m also guessing by the WOA barrel choice that accuracy/repeatability is a requirement or at least a factor?  Look into what your needs are and see if a LW profile barrel can perform.  A lot of pencil type barrels will tend to open up groups a bit once you get some heat in them (some more than others).

Tried a titanium gas block once. Huge difference in weight on the front. Expensive but really makes a difference in swing speed.

I’ve found that the “M4” profile barrels found on most factory rifles like the Bushmaster Optics Ready Carbine seem to be light, accurate, and don’t wander zero after a shot string.

They sell a “QRC” lightweight upper as well with a 1/8 twist Nitride barrel and pinned gas block.

I have one and am very impressed by its accuracy and weight. With a titanium suppressor it really balances well.

BUILD