subsonic 300 AAC BLK (Blackout) load recipe

The BLK was designed to accommodate both supersonic and subsonic ammo without any tuning or performance headaches. The subsonic loads are intriguing: who wouldn’t want to send a 220gr Sierra HPBT for a big, heavy projectile to create high performance at subsonic fps. That’s a mere 3.4 times heavier than common 62gr 5.56 ammo. 5.56 ‘heavy’ projectiles weigh in at 75-80gr. The BLK was designed to run suppressed too- so if real quiet os your bag and you I enjoy some closer range shooting (effective hunting range is about 100m). 

This is the recipe I use for supersonic loads for my Model 1 300 Blackout Carbine. About 50 rounds of this recipe fired through my 16″ rifle at distances up to 200 yards to punch paper and plink. I’m using 220gr Sierra Match HPBT for the projectiles and AA1680 for powder. The bolt locks open after the last round clears.

Accurate 1680 is tough to find sometimes but the powder density matches the case capacity very nicely in the BLK. rI use CCI primers because they are a bit faster burning than Winchester and that’s handy for subsonic loads to help ensure you maximize ignition to help cycle the action. KB ALERT: Winchester or a similar, harder primer are the only safe ones to use in a Lee Progressive loader-be sure you use the correct type of primers with a Lee Progressive system.

I run this load recipe on my Lee Precision 4-hole Turret Press and load both factory BLK brass and 5.56 brass trimmed to 300BLK specs. 

NOTE: Users assume all risk, responsibility and liability whatsoever for any and all injuries (including death), losses or damages to persons or property (including consequential damages), arising from the use of any data, whether or not occasioned by publisher’s negligence or based on strict liability or principles of indemnity or contribution. kR-15.Com neither assumes nor authorizes any person to assume for it any liability in connection with the use of any data.

Caliber: 300 AAC Blackout (300 BLK) 
Projectile: 220gr Sierra Match HPBT
Primer: CCI Small Rifle Primers (no. 400)
Powder: AA1680
Measure: 11.2gr 
Lee Auto-Disc setting: 0.71
Min OAL: 2.140
Max OAL: see discussion below
Regarding OAL I had to experiment to find a length that fed well. Here is my thought process to arrive at the OAL I set my dies to:
This seems like a pretty big tolerance. I set my seating die to give me the longest possible cartridge-about 2.160-2.200. Some factory ammo I purchased didn’t feed well due to FTC (Failure to Chamber). It turned out that they were the short end of the SAAMI OAL specs. Some scouring on forums and experimentation proved that longer OAL fed better : no FTCs on any of my BLK cartridges since this change
The beauty of using these high mass projectiles in a modern sporting cartridge is that they pack a serious punch and don’t use much powder– a nice bonus when supplies are scarce or you’re on a budget. Did I mention that the BLK delivers the same amount of muzzle energy from a 10.5″ barrel as a 5.56 does out of a 16″ barrel? 

10 thoughts on “subsonic 300 AAC BLK (Blackout) load recipe

  1. Foghorn

    So, yeah. You’re kinda using a picture of my rifle as the banner up there. And you know what? That’s cool. Let me know if you want a better view.

  2. Pingback: Internet reloading recipes- a cautionary tale | kR-15: info and resources for firearms enthusiasts

  3. rbcadd

    I’ve also had major problems with failure to chamber, regardless of the bullet used or OAL.

    Cases were trimmed to 1.358, the middle of the tolerance, and that specified for the Barnes 110 GR Tac-TX, and at least one commercial re-formed case producer I’ve talked to.

    I plan on using the Barnes 110GR Tac-TX, the Sierra 110 GR Varminter HP, the new Sierra 125 GR OTM FB Matchking, and the Sierra 220 GR Matchking for subsonic.

    I’ve been through two sets of dies, first the Lee dies, and the latest being the RCBS small base dies. Same problem. Producing more failures to chamber, than not.

    I’ll get one or two round that feeds and chambers perfectly, and then get 4-5 that feed, but won’t chamber, and seize up hard in the chamber. A real booger to get out without breaking anything. all rounds seem to feed well, but fail to fully chamber and lock the bolt.

    I’ve used PCT go and no-go gauges to check headspace, as well as the Wilson case gauge, all of which checks out, and says everything SHOULD fit.

    I’m about at my wits end trying to figure this one out.

    Any ideas?

    1. k4R-15 Post author

      If this were happening to me I would post a question on and in the BLK forum but here are my initial thoughts:

      Are you making your own brass? Otherwise what’s the source?

      1.358 is below min cartridge length shown here

      Is there any variance in the cartridges length? I would check every dimension on enough loaded rounds to see if there is an anomaly.

      Also did you rebarrel an existing upper or purchase a complete upper?

      1. rbcadd

        Thanks for your quick reply.

        The brass was cut down from some once-fired PMS .223 shells I had laying around forever.

        The SAAMI cartridge length specification calls for 1.368 +0.000 -0.020, so the lower limit would be 1.348. 1.358 is right in the middle. As I said, Barnes calls for 1.358 shell case length for their highly regarded 110GR Tac-TX.

        So I milled the brass down to 1.358 +- 0.001, deburred, and cleaned it..

        I bought set of Lee dies, and set about carefully reforming it. Then I checked each one in a Wilson case gauge. Every one of them appeared to be within spec.

        When I was getting a 50% failure rate chambering, I went and bought a set of RCBS small base dies, thinking the cartridges just needed to be sized smaller than the Lee dies, and then fire formed to fit my chamber.

        But I was STILL getting a 50% failure rate, same problem. Major frustration was beginning to set in. All the YouTube videos I watched and articles I have read made it seem SO MUCH easier than what I was experiencing. Something was definitely wrong. So I did some more research.

        It turns out, after going to the SSK website, where they have lots of .300 Whisper info, that they warn against using re-sized .223 brass, because the thickness of the brass at the neck varies wildly, and if it is too thick, the rounds may feed, but won’t chamber and will get jammed.

        Sounded VERY familiar.

        That is the exact problem I am having, so I checked the thickness of all the remaining re-sized case mouths, and sure enough, at least half of them were WAY too thick. If you have a .308 bullet, seated into a cartridge with 0.015 thick brass at the neck, then you end up with .338 outside diameter at the case mouth, when it should be .334 max. Houston, we have a problem.

        So I then sorted through and separated out all the brass with a thickness of 0.013 or less, loaded them up, and, lo and behold, every one of them feeds and chambers perfectly. Problem solved. [Insert big sigh of relief here]

        Make sure and measure the thickness of the brass at the case mouth BEFORE you go to all the time and trouble prepping and loading it. I would have saved me many hours, components, another set of dies, and lots of frustration, had I known. Now I know, and so does anyone reading this. Hope this helps others avoid this problem.

        Thank You very much.

      2. k4R-15 Post author

        That’s great news rbcadd– thanks for the detailed writeup. I purchased some factory brass and some trimmed and resized from to get started (hard to do nowadays since 5.56 components are all going towards ammo).

        Eventually I will end up making some brass so this is helpful for me — I also help others can benefit from this information.

  4. rbcadd

    The only question I have now, is what percentage of a given batch of re-sized 5.56 brass typically has a neck wall thickness of more than 0.013? (0.013 + 0.308 + 0.013 = 0.334). If it is a large enough percentage, then it becomes a major time and money factor, as anything thicker than that is very likely to be unusable, unless a person wants to go to all the trouble of turning down case necks, one at a time, til they die.

    I’m not an obsessed competitive bench rest shooter, so turning case necks has less than zero attraction for me as a solution. I just want to re-form, re-load, and shoot the darned things, with high dependability, and reasonable accuracy, of course. If I can reliably hit an apple or a pinecone sized object at 100 yards, I’m a happy camper. A medium cantaloupe sized object at 400 yards is a thrill. Turning down case necks is not my idea of fun. Making things go boom and splat, in short order, and repeatedly, is.

    1. rbcadd


      I am informed by Brad at, a maker and seller of quality 300 Blackout re-sized brass, that foreign brass tends to be too thick in the neck area, so avoid it like the plague. He says domestic brass, made in the U.S.A., is consistently thinner so he never has problems arriving at or slightly below the 0.334 neck diameter specified by SAAMI for the 300 AAC Blackout.

      I had several handfuls of Korean-made PMC .223 brass laying around, so I thought I would get dialed in on that before buying LC brass. My mistake.

      A lesson and word to the wise: Don’t waste your time trying to process and re-size thick foreign-source .223/5.56 brass for 300 AAC Blackout. Stick to the domestic stuff.

  5. Pingback: Chronograph results- at long last | kR-15: info and resources for firearms enthusiasts

Leave a Reply

Please log in using one of these methods to post your comment: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s