Wildcatting, the practice of making interesting and unconventional cartridges together, is one of the attractive qualities of the 300AAC Blackout offers. That magnificent achievement (insignifia of the day: 300 WTF was its nickname before AAC finished the BLK project) is capable of sending a diverse range of projectiles downrange- 110gr to 225gr is typical. Compared to .243 Winchester, which typically ranges from 55gr-117gr, it's a big difference.
Another reality of owning and feeding a 300 BLK is that while manufacturers are continuing to add 300BLK product lines it's hard to find much besides 220gr HPBT or 110gr FMJBT rounds. That commercial stuff is pretty expensive too- limited availability pushed ammo into the $1-2 per round price range last time I checked. So why not load your own 300 AAC ammo and embrace the diversity of options and lower prices?
Well, handloading is an inherently dangerous activity if you arent really careful. Your ammo is only as trustworthy as the data (aka recipe) you use to determine whether or not a cartridge is safe. Fortunately it's typically not difficult to find reliable data- that's why there are multiple books published, Quickload software, and for the really brave/lazy people, the interwebs.
The BLK is still pretty new by firearm standards… My Lee Precision Loading Manual doesn't have any BLK data. The dies I got from Lee (they make a great product for the hobbyist and sell at a very fair price compared to their competitors) came with some data but it doesn't cover all the different projectiles you can find in 7.62 NATO/.308 – not even close. Now what? Google is my pal. I search for everything and crowd-source info. The limits for your recipe should be constrained by either published data or from a reliable piece of software like quickload. Here is a scary example of why I look for a credible source to add confidence to load data…
Recently I switched my loader from subsonic to supersonic settings to pound out some rounds. One small problem- I didn't adjust the bullet seating die as far as I should have, and the subsonic projectiles seat a lot lower and logically use a lot less powder. I didn't notice until a few cartridges seemed notably short… and I googled a recipe and according to it I was at a safe COAL (Cartridge Over All Length). It must be confusion because Its been a few months since I ran this load, right?
I was unconvinced…being cynical is a good trait for this sort of work. So I took a second look at my notes and measured a few rounds (3)left over from a range trip as check measurements. The internet load had a way shorter OAL(Over All Length- same as COAL) than a recently published load by Accurate Arms (links to the bad data and good data are below) and I trust them since they manufacture the powder and therefore have a lot to lose by publishing recipes that aren't checked for safety.
Thank goodness I followed my instincts. Check out the pressure graph I found from a quickload user on blktalk:
In case this is the first time you have seen one of these graphs, note how the red line makes that lovely crest above the dotted horizontal line labeled 'Max Pressure'? See how that dotted line is preceeded by a reddish zone under it? That's very bad- as in KB (ka-boom!). Also red bars and the word DANGER in all caps in the upper right corner indicates that you don't want to put these rounds anywhere near a chamber.
This powder and measure really fills the case, so I wanted to be sure that nothing measured up short. I was happy to remove and dismantle the 4 or 5 that were below Min OAL of 2.140nafter remeasuring. I actually have had the best feeding performance using longer COAL. I gradually increased the OAL until I found a setting that fed more easily.
Longer OAL doesn't increase the pressure so as long as the round is shorter than max oal fits in the mag and cycles the action you're good to go as long as you don't seat the bullet too short on the powder and create excess pressure – aka anything under Min OAL.
The other limit for length is max OAL to ensure your projectile or bullet doesn't get lodged past the chamber- that is also unsafe and can cause dangerous chamber pressure. Use the SAAMI specs and the pressure limit/published min OAL checks as your safety constraints.
Brain break while that last part soaks in…FUN BLK FACT OF THE DAY: the reason subsonic 300 BLK loads cycle the bolt is because the projectile is heavy. For this reason most 5.56 subsonics won't cycle with a standard gas system.
The Reliable 1680 Load Recipes
The data I trust and have used successfully in the past is published by Accurate Arms– they make the powder so I trust their recipes. Also it worked safely in the past.
The Unsafe Load Data that should NEVER be used
This file came up high in search results for 150gr OAL data – IT IS UNSAFE AND SHOULD NOT BE USED. The min OAL listed for 150 gr FMJBT is way too short for 1680 powder here. It's a nice looking document but if the 1680 data is that wrong it's not trustworthy. Also the parent domain has nothing to do with firearms or reloading – further making the doc suspect.
Be careful with your recipes- safety should always be the first priority.