According to the metrics this site gets more than a couple visits from people looking for load recipes and info on components for 300 AAC Blackout cartridges. It seems that 1680 is even more scarce in the wake of the biggest gun and ammo buy in history – so I will do my best to assist.
DISCLAIMER: Use this information at your own risk. Just because these settings work for me (or in some cases below someone claimed they worked in an internet post) and my rifles it does not guarantee that they are safe for you to use—in no way can I guarantee that these recipes won't injure or kill you. I strongly advise reducing your starting load and working your way up.
I also strongly advise using Quickload or something similar to ensure safety when experimenting with load recipes. Ammoguide.com is a pretty nice subscription-based site I recently joined that has vast resources, tools, load data and info. Handloading is a dangerous activity and mistakes can have serious consequences- picking little shards of aluminum out of your face, losing fingers, even death. Please be extremely safe and extra cautious while reloading and proving out recipes.
Accurate Arms 1680 is nice for the BLK because the powder charge matches case volume nicely- not a ton of space inside a cartridge…. 1680 is a popular wildcat and 5.56 powder also though– a few years ago 5.56 subsonic loads were a popular home recipe (and still are although it's tough to get a standard gas system to cycle the action and still stay below 1100 fps velocity in a 5.56). 1680 is a decent powder for supersonic 5.56 handloads also– it's typically not easy to find, especially in the current market climate of constrained ammo and components.
Fast burning low volume rifle powders are a must for the BLK and even some pistol powders can work- while this is traditionally viewed as dangerous since pistol powders burn too fast for rifle loads the AAC Blackout is an exception. Not only is the case small to begin with but once you add a .308 projectile the room for powder gets scarce.
Another thing to consider is the projectile you are loading. The two I stick with most of the time are 220gr HPBT (Sierra) and 150gr FMJBT- if you need to know that projectiles should be .308 caliber you probably should refer to the disclaimer above, stop reading this and learn about the 300 AAC Blackout cartridge and maybe about basic handloading.
110gr is supposed to be the ideal projectile for performance and range in The BLK. I am a subsonic fan though so I prefer the 205gr and higher weight projectiles since they are easier to keep under the 1100fps threshold. The 150gr FMJs are more economical for range loads- at least for copper jackets from Barrys. Projectiles can make a big difference though when it comes to powder.
Powders that cover most 300 BLK projectiles
Hogdon H110 works well for 110-220gr projectiles. Note: H110 loads should never be reduced by more than 3%. Tuning any lower can result in inconsistent burn- aka squib loads- aka projectile in the barrel just waiting to become a KB.
Hogdon H110 isn't bad according to Foghorn (who graciously loaned the header photo you see on this site).
Hogdon Lil Gun and IMR 4227 also have good coverage across the realm of projectiles for the Blackout.
Powders for specific 300 BLK projectiles
IMR Trail Boss is an interesting critter. It was designed for Cowboy Action pistols but is one of those pistol powders that performs well at high density and voila! Is a handy charge for the BLK. My data suggests that Trail Boss is better for the smaller projectiles (110-115gr). Bonus: Trail Boss is the proverbial cat's ass if you're looking for subsonic 110-115gr loads.
Reloader 7 (R7) also has a good reputation for subsonic 208gr and heavier projectiles.
Unique is another powder that is popular with the Wildcat crowd. Some people tell me it's ultra frustrating to work with and others rave at the versatility of the stuff. I have no experience with this one so you will have to scour the forums for load data. Try 300blktalk.com to get started.