I've been looking to pull the trigger on one. Does anyone have any recommendations for the lightest available? I've been looking at the 2A Xanthos but the upper doesn't have an FA. Any recommendations?

Most lightweight uppers have no forward assist, it makes them lighter.

Not a lot of options for small frame guns yet, not sure if there is any one selling receiver sets. The Xanthos is a large frame.

Yup, Xanthios is large frame. And you don't need a forward assist. If there's a problem, just clear it and start over.

I'm so confused about the differences.

Haha, me too. I haven't put in the time to learn what's what on the AR10 market. But the good news is that 2A makes all the parts to build a complete rifle so they can sell you all the right stuff.

There are no small frame receiver sets on the market at the moment.  

There is a lot of variation on the large frame guns that prevents them from being completely interchangeable, but they're all about the same size.  

There's the original AR10, The Knights SR25, The Armalite AR10B & AR10A, The DPMS LR308 with high or low upper, the POF P-308, and the Bushmaster/RRA LAR-8.

Small frame guns are the DPMS G2 and the Adam Arms SF 308.  These are pretty much interchangeable with the DPMS being DI and the AA being piston.  Mega and POF have SF models in the works, no telling if anything on them will be compatible with the DPMS and AA.

With a few exceptions they at least all use the same mags.  The Armalite AR10B was introduced during the AWB so it was engineered to use modified M14 mags which were readily available, whereas AR10 mags were virtually non-existent.  The RRA LAR-8 was originally designed by Bushmaster, also during the AWB.  It was designed to use FAL mags.  Bushmaster dropped it after only producing it for a very short time.  RRA then bought the design and is producing it today.  The LAR-8 shares virtually no interchangeability with any other patterns.  Current Bushmaster .308s (along with all other Freedom Group .308s) are DPMS low pattern.  There were also a few DPMS based variants from a few different companies that used G3 mags, I don't know of any of these being available today.  

With the notable exception of Mega Arms, most of the aftermarket receiver manufacturers are using the DPMS high pattern.  Mega uses the Knights SR25 pattern.  The only significant difference between the Knights and DPMS patterns is the cut at the rear of the receiver, DPMS is round, Knights is angled.  The POF P-308 is very similar to the DPMS pattern but uses different takedown/pivot pins.  

Armalite uses a different bolt depth specification, different barrel nut thread pattern, different barrel extension diameter, and a different BCG design (uses a spring loaded firing pin instead of free floating).  There is overlap with the bolt depth, so it's entirely possible to get proper headspace using a DPMS barrel with an Armalite BCG or vice versa.  It's not recommended and if you do decide to do it, make absolutely certain that you do check headspace prior to firing.  BCG parts are not interchangeable between the two, you will need to use an entire BCG from one or the other.  DPMS barrel extensions will be slightly loose in Armalite uppers and Armalite barrels will be tight in DPMS or KAC pattern uppers, but they can be used.  DPMS barrels in Armalite uppers will likely require the use of shims.  To get an Armalite barrel in a DPMS or KAC upper you will probably need to heat the upper and freeze the barrel.  

Handguards are interchangeable between KAC, DPMS high, and Armalite, but the Armalite will require a different barrel nut.   DPMS low uses the same barrel nut pattern as the DPMS high and KAC, but the top rail is the same height as an AR15, so slightly lower.  

The 2A is a DPMS High pattern.

There are 3 different rail heights with the DPMS LR-308 and AP4 Gen 1 rifles, and a 4th with the G2.

IIRC, the design was by RRA and then licensed to Bushmaster. RRA fixed some of the issues, renamed it, and continued to build them under their own name after Bushmaster decided to stop marketing them.

Just looking at the numbers, the lightest I see is the DPMS G2 Compact Hunter with a Rifle-length Lancer carbon stock, Magpul slim grip, and whatever V7 lightweight AR15 parts will fit.  That should be around 6.75lbs.  You could further drop weight by getting an adjustable gas block and running an empty buffer, or making a custom Taccom buffer.  You might be able to get lighter by running a Vltor A5 pistol tube with empty carbine buffer and ACE skeleton stock, no pad.  But I like the lancer carbon rifle stock, nice cheekweld.

That is correct.  Bushmaster was licensed to produce it for them.  When the licensing agreement ended, so did production of the BAR-10.

I sold a lot of them working at a LGS at the time out East, thought it was a great option at the time due to the ban, in addition to the excellent bolt hold open and release controls.

Compared to what is available now, and that horrendously long receiver set, the thing is an albatross if you want to carry it and aren't 6'7"/250 lbs +.