Note: We all know the .308 is not doable in the AR-15, and the unnamed authors later highlight that fact.
Many AR-15 owners begin to consider different cartridges after they’ve built or bought a few rifles in the standard 5.56x45mm/.223 Remington chambering. America’s love for .30-caliber cartridges is strong, and AR owners are not immune to the lure of a bigger bore.
The .308 Winchester has been around for years. The 300 AAC Blackout is more than a decade old and has become quite popular, as well. Both cartridges can be chambered in AR-style rifles. However, there are some significant differences when pitting the 300 Blackout vs .308 Win. that can impact which you should choose for your next project.
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Man oh man... You're opening a can'o'worms here. Pardon my rant...
I'll start by explaining that I admire genius when it is reflected in a firearm design, no matter what form factor one is addressing. The 1911 pistol, the Glock 17, numerous rifle and shotgun designs by JM Browning and on it goes, including Eugene Stoner's AR design. What some (many) fail to recognize that the AR-10 scale design existed first, intended to produce a rifle which handled the 308 cartridge in order to directly compete with and replace the M14. When the powers that be decided they wanted a small caliber cartridge with a significantly higher velocity than the 308. Eugene & co went into high gear and scaled down their AR-10 to fit the proposed new cartridge, and in a fairly short period of time the 223/5.56 and the AR-15 were born. The important thing to recognize is that EVERY dimension of the AR-10, followed by the AR-15, were meticulously scaled to fit their respective cartridges, the 308 in the AR-10 and the 223 in the AR-15. The length of the receivers, the length and diameters of the bolt carriers and the diameters of the bolts and bolt faces were designed to have "just the right amount of meat". Where those who follow err, in my opinion, is in trying to shoehorn cartridges with "just a little more diameter" and/or "just a little more length" into an action that was not designed and scaled to handle the increase in dimensions. This results in things, for instance, like thinner "walls" around the bolt face and less support for the locking lugs when fitting "fat head" cartridges like the 6.8 SPC, the 6.5 Grendel, and the 6 ARC into a standard AR-15 action by swapping in an altered bolt, magazine, and of course, an appropriately chambered upper. It can be made to work (for a while), but issues will soon rear their ugly heads.
The 300 Blackout (or 300 AAC or whatever other monikers it has worn) is an example of the reverse situation: fitting a cartridge of the same case head diameter as the 223 round, but a little bit shorter OAL for the round. This only requires (usually) a tailored magazine to handle the slightly shorter case plus an appropriately chambered upper. The bolt and the carrier can remain the same. However, the 300 BLK is notorious for requiring quite a bit of tinkering with action springs and buffer weights in order to get it cycling reliably. Introduce a suppressor and/or a "strange" barrel length / gas system length that varies from the 300 BLK "formula" and frustration may (and probably will) ensue.
All of which is to say that in a semi-auto or full auto firearm, design is EVERYTHING. When something has been around as long as the AR-15/M16/M4, the bugs get worked out and everyone kind of "knows" what the formula is to get the system to run reliably and what the expected service life of the critical parts will be. Go to dinking around with that "formula" by introducing a "shoehorned in" cartridge and the "formula" is no longer valid, and the service life of the parts will probably not meet expectations.
All that said, the 300 BLK is a specialty cartridge which happens to fit OK within the constraints of the AR-15 design. It has now been around long enough that the "formula" which will make it work is getting pretty well ironed out and more commonly known. When I say that it is a "specialty cartridge", I mean that it's designer never intended for it to compete with the 308/7.62. Each performs a specific set of tasks within its respective AR platform, and ne'er the twain shall meet. Need a subsonic round that suppresses well in a lighter platform that can also perform at or near 30-30 Winchester levels when pushed to supersonic velocities? The 300 BLK in the AR-15 platform is your huckleberry. Need a harder hitting cartridge for ranges out to 1000 yards, maybe a bit more? The 308/7.62 in the AR-10 is what you need. It will not easily be reduced to subsonic suppressed whisper quiet levels, however. Not in an AR platform.
Step 1 - decide what shooting task you wish to perform
Step 2 - choose an appropriate cartridge that will readily perform that task
Step 3 - choose your hardware that is appropriate to the task at hand and the selected cartridge
Step 4 - Avoid the "Don't force it, son. Get a bigger hammer" syndrome and stop trying to mix cartridges and actions that weren't designed to work together.
End of rant
There. I feel better now.
escape mode