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Have you considered the route I took? A road far less traveled. You could use my concept but tweak it in the other direction (for a lightweight 6.8 build). I wanted a heavy barrel without heavy barrel woes (meaning it's so heavy it's tiring, slow to present, front heavy etc.) and to increase overall system performance at the same weight.
So I paired my 16" Daniels S2W CHF CL 5R SPC II barrel with a JL Billet NANO Ultra Magnesium / Aluminum 15" handguard....the handguard with hardware is just 6oz!!! It's lighter weight than carbon fiber handguard setups in that length that clock in at 8oz total. They also make a 13" version that's even lighter yet if you don't need full coverage of a 15".
A standard 6061-T6 15" handguard will run north of 15oz-16oz even with an aluminum barrel nut like the Geissele 13.5" SMR on the SF's new URGI. BCM's 12" m-lock handguard still clocks in at 11oz despite being considerably shorter as well.
So my 16" Heavy barreled 6.8 SPC upper weighs the same as a 16" gov profile 5.56 upper with a standard 15" 6061 T6 m-lok handguard....actually the balance is even better with my setup as more of the total weight is back at the receiver due to the S2W profile and mass bias of the handguard. I've not seen ANYONE explore this concept.
The only use cases I've seen for AZ61A alloy is on ultra lightweight builds where the idea is "light as possible everywhere". No one has used it to off-set the weight of a heavy barrel to make an "standard weight" high performance upper that punches well above it's weight class.
So, what if you paired an ultra lightweight handguard with a standard scout profile 6.8 SPC barrel? That would give you a lightweight 6.8 SPC setup without compromising thermal related accuracy loss associated with pencil barrels and the faster throat erosion that also accompanies those profiles. Throat erosion is directly related to heat, the hotter it gets, the softer the steel and the faster the rate of wear on the lands, the issue is worse with nitrided steel vs. chrome lined.
JL Billet's handguards will fit the SA piston gas system if your bent on that still, but I stand by synopsis to avoid it. I think your better off using DI NiB bolts and NiB carriers in a traditional DI system if your concerned about maintenance time as they are pretty easy to clean and all your going to do is shift the cleaning from the receiver to the gas block / piston assembly.
Short stroke tappet piston setups make the most sense in heavy full auto applications where heat can become a real issue. For a typical infantry AR use, LE or even military, DI with modern plating and good mags is very reliable even if you don't clean it, just keep it wet! BCM ran a mil-spec DI upper for over 30k rounds....not cleaned even once. They just kept dousing it with lube...Fail Zero with their Nib setup has run over 50k rounds through their test upper WITHOUT lube...that's a couple barrels at least...
Here's the weight breakdown difference for illustration (adjusted barrel weight for a 16" gov profile, not the 14.5" used on the actual URGI to give an apples to apples comparison):
URGI (Upper Receiver Group Improved, modified with a 16" gov profile vs. 14.5" gov profile for an apples to apples):
Geissel 13.5" SMR M-Lok Handguard 14.9oz
Daniels 16" Gov profile mid-length 556 Barrel 27.2oz
Total: 42.1oz
6.8 URGHB (Upper Receiver Group Heavy Barrel, this is my implementation):
Handguard 6oz
Barrel 36.9oz
Total: 42.9oz
Difference 0.8oz...and more of that weight is at the back of the receiver where the weight matters a lot less than out front. But I have a barrel in a more powerful caliber and in a heavy profile....my 6.8 URGHB would massively outperform their brand new URGI at nearly the same weight and handling.
6.8 URGSCOUT:
JL Billet NANO Ultra Mag / Alu 15" Handguard 6oz
ARP 16" Scout Barrel 28.6oz
Total: 34.6oz
Or if you pin the gas block AND if ARP still has them:
6.8 URGSCOUT14.5:
JL Billet NANO Ultra Mag / Alu 13" Handguard 5.8oz
ARP 16" Scout Barrel 25oz
Total: 30.8oz...that's a pretty lightweight barrel / handguard config for 6.8 SPC.
Even my 5.56 Daniels Light Weight CHF CL barrel clocks in at 24oz...and that's a pencil profile I use for a training upper / CQB upper for an alternate caliber to my 68 that's a primary.
Gas piston system will also add more weight than a DI gas system, even the SA one (lightest piston system of them all) is 3oz more total weight mostly out front than a low profile gas block (1.3oz) and gas tube (0.6oz) in a DI gas system. Carriers are the same weight. So if your weight conscious, go DI. And this coming from someone who ran AA's also reasonably lightweight piston system for years....
Note that I have since replaced the bulky picatinny mount surefire pressure pad with sure fire's smaller tape version and magpul m-lock mount plate for a lower profile and cleaner design, no rubber bands or zip ties, but at the time of the photo worked reasonably well still, even if a bit ghetto, I was using what I had at the time of the initial build. That's a heavy barreled 6.8 barrel with JL's mag / alum handguard.
Handles like the oh so typical 16" 556 gov profile / 6601 15" m-lok handguard combo but shoots in a league of it's own comparatively.
P.S. sorry for the uber lengthy post, but there was a lot of technical reasoning to my suggestions. I've tried MANY configurations over the last 10 years and I've really honed in on what works and what doesn't, what's the best and what's marginal. I'm always after overmatch, not just in caliber selection, but rifle design, so I roll my own as they say.
