Have an old AR10 lower left and thought about building a medium weight, carbine style, WV brush gun.  Also considering just selling it and building a .458 SOCOM too, but that's not what I'm here about. I see Armalite has a .338 Fed upper made to bolt right on.  No one, that I can find, makes a .358 Win upper and the ammo isn't that common.  Problem is, I like the ballistics of the .358 a lot better and I've read that the .338 Fed likes a longer barrel than the .358 and I'm trying to keep it under 20" (18" I hope).  

My plan for the rifle is a MOE stock (maybe a cheek riser), lightweight free float tube, no BUIS,  Weaver classic 1-3X scope on weaver rings.  Other little things include ergo grip and some sort of forward facing compensator (not sure on this yet as I'm still deciding caliber and not all devices are available in all calibers).  Using 10 rnd mags, I don't think this rifle should weigh more than 9 lbs, right?

If I go the .338 Fed route, I could get a complete upper from Armalite for $800, but could I cut the 22" barrel down to 18" (and threaded), and keep the ballistics necessary for the available rounds to perform correctly?  I'm pretty sure an 18" barrel with a rifle length gas system would cycle, but I'm not even sure on this.

Here's the unique route; could I take a medium contour carbine .308 barrel and have it "rechambered" for the .358 win?  Just start from scratch with a custom barrel blank?  I'm totally assuming the bolt is same and would not have to be altered or replaced.

Both rounds would satisfy my needs/wants, but the .358 Win just does it a little better.  Just curious what you guys think.  Appreciate any advice.

ETA: I will not be reloading for this.  It will be a purpose built gun pretty much used 3 or 4 weeks a year deer hunting and will not see many range trips.  Only factory ammo will be utilized.

BTT

Anyone with some insight on either of the two calibers?  Should I take this to a different forum?

I'm aware of an FAL in 358 Win, but I don't know if that bullet diameter difference from 308 translates effectively into an AR-10.  The slightly larger bullet diameter might cause feed issues, but it might not.  

I think the bigger issue is money.  If you're willing to pay for the barrel blank, extension, and chambering, then a 358 is essentially no different than any other 308-based round.  You asked if rechambering a 308 tube would work;  you'd have to rebore the 308 to a 358, and although I suppose it can be done, I can't imagine it's anywhere near cost effective.  Redman reboring prices start at $400 and go up.  That doesn't sound like an economical route.  It would be cheaper to start with at 358 blank, methinks.

You mentioned a 338 upper from Armalite for $800.  I'd hazard a guess that the work for a 358 tube alone would cost you near that amount, but if you're dead set on a 358, then by all means let 'er rip.

However, for deer hunting, I don't see why a 308 wouldn't serve you just as well.  If you need the 358 for the cool factor, I understand, but just be aware it's gonna cost you an arm and a leg.

I've got a 338 Fed and have been pleased thus far.  It is on a DPMS style receiver set but same idea, have an 18" barrel, Carbon Fiber handguard and while I haven't ran it through a chrono though so I can't comment on how well the 18" is actually performing.  I can tell you it cycles fine using a rifle length gas system.

I have a DPMS 18 inch .338 F barrel and it does just fine with that tube.  IIRC I am getting over 2400 fps with 185 gr. TTSX bullets and accuracy is good too, coming in at about 1.5 MOA but I have not done a lot of work turning it yet.
If you want a .358 Winchester barrel for the AR10, buy a blank, barrel extension, and maybe reamer and .308 headspace gauges (same for the 243 Win, 260 Rem, 7MM-08, 308 Match, 338 Federal, 358 Win) and have Paladin Machine Service (803-475-7542) make you a barrel.  It will cost you no more than $500 total, or about what a Noveske barrel sells for.  You can also rent reamers and headspace gauges from 4d Reamer Rentals  However, check with Thomas at Paladin because he may have the reamer already and I can almost promise you he has the headspace gauge already.
By the way, the .358 Win in the AR-10/LR-308 platform has been done and it works just fine.