A 6920 is more than enough rifle for 99% of shooters dealing with 99% of shots taken with an AR-15. A friend of mine can shred 3-gun courses with a 6920, and makes dudes with multi-thousand dollar rifles look like boys playing with twenty-twos. The indian, not the arrow. That said, maximizing the platform, when you have the luxury of doing so, makes a lot of sense. If you have the money to buy, and/or the time & inclination to build, you are confronted with making a series of choices, that of gas system and rail being two of the more significant ones.
For me, gas system is a an engineering choice first and foremost. I prefer the longest gas system that I can get in the rifle while maintaining long enough dwell time to reliably cycle the weapon. Typically, that is only a couple of inches of barrel beyond the gas port. From what I can tell from reading a vast amount of other people’s experience with the topic (I don’t own 30 rifles of varying stripe), when the amount of barrel beyond the gas port drops to below two inches or so, the rifle can get squirrely depending on ammo, gas port size, etc. That said, a 7.5 inch carbine gas system lets a rifle get down to that mk18 10.3 barrel length and still reliably cycle the weapon.
As to non NFA rifles, be it a 14.5 pin&weld or a 16+, a carbine gas system leaves so much barrel beyond the gas port–and especially in the early, hot/high pressure section of the barrel–that the port pressures are significantly higher than what is required to cycle the weapon. Mid-length gas systems on non NFA rifles with barrels under 18" (which should have a rifle length, probably) allows for better operating conditions for the weapon.
Next, free floating gives you a ton of extra room to mount garbage on the rifle. Such long rails also allow you to grip farther out towards the muzzle, which aids in controlling recoil and, for me, driving the muzzle from target to target. They also allow for the attachment of bipods, which is crucial to milking maximum accuracy out of the rifle at longer ranges. The same friend as mentioned above is able to make on-demand sub-moa hits with a 556 rifle out to and beyond 600 yards.
I am not sure if he could do that with a stock, non-free floated 6920, but then again, you never know till you try…
When I bought a new rifle, I didn’t have restrictions from an agency regarding what I could have. And so, I bought mid-gas in a 16" barrel. At the time of my purchase, I could have gotten a plastic handguard and FSB (though these were significantly rarer in manufactured complete rifles then). But, as I had control over the whole process, I ended up with a Troy Alpha rail installed on the rifle from the factory. So, factory free-float mid length gas.
I am putting together a 14.5 rifle now–which rifle will be getting a thread in the Custom Builds section whenever I get the damned Geiselle rail (ordered 05/25, still “processing”)–and again faced the same questions. I could have just bought a 6920 and moved on with my life. Instead, I decided that I wanted to waste a bunch of time and money learning how to do armorer level work on the AR-15 platform, and fully assemble my own rifle. Given that I had complete control of the variables, and that my barrel length was 14.5, it made no sense to get carbine gas. Mid length, as stated above, makes for a better operating system. Next, I could have easily just gotten a magpul handguard, but since I had complete control over the variables, I’d gone with a low-pro gas block, and ordered a gucci-ass G rail. The end result will be a rifle that has nearly all of the slack taken out of the system: the only variable left will be me.
