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300 Blackout Bolt Gun SBR Loads

NHPiper

NHPiper

Tactical Bagpiper
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Minuteman
    • Feb 17, 2017
    2,551
    1,052
    Concord, NH
    I have a 16 inch Wilson 1–8 twist barrel that was cut down to 9 inches. Previously I ran 8.1.gr of N110 behind a SMK220 in Starline brass with a CCI small rifle primers. There was about a 0.030” with the bullet. This gave me very good accuracy, ragged, one hole groups at 50 yards with a velocity of 970fps, however, the SD was higher than I wanted at 12–14.

    The barrel was recently cut down to 9 inches. I didn’t seem to lose any velocity and the SD remains the same, so I tried charges at 8.3 and 8.4. 8.3 shot much better over the chronograph with a velocity of 1001fps and an SD of 5. However, accuracy sucks. I’m now getting 2 inch groups at 50 yards. I tried messing with the jump to run it between 0.02–0.040” and it didn’t seem to make a difference. I also cleaned the barrel to make sure that there wasn’t debris from the machining.

    The torq seems correct on all of the parts, and I am running the action screws at 50 LBS.

    Is anyone else hand loading for a similar rifle with subsonic bullets? The idea behind this gun is to push the boundaries with subsonic loads out past 600 yards, so I need my ES to be as tight as possible while delivering sub MOA groups.
     
    I am also working on a .300blk bolt action (Howa Mini) subsonic load, except I am sticking with the original 16" long barrel. Currently trying out about 10.8gr of A1680 with Hornady 208 ELDMs in Starline Brass with Federal 205m primers. I've been getting about 1050fps with SDs around 20 and 1MOA accuracy. I'd like to tighten up the SDs, but the accuracy is pretty decent at the moment all things considered. I'm thinking to try the SMK 220s next and see how the additional weight affects my load and case capacity as I feel like that is some of the issue with the high SDs.
     
    I'm running a 16" 8 twist Bartlein in my 300 Bo bolt.
    Running 190 smk's using 9.2gr H110, 6.4 sd, 1038 fps, consistent at 600 yds
    14.6 mil to 400 & 26.3mil to 600

    I tried 220 smk's but never got great groups with them, 1026 fps, 9.5 sd.
     
    awp762 said:
    I'm running a 16" 8 twist Bartlein in my 300 Bo bolt.
    Running 190 smk's using 9.2gr H110, 6.4 sd, 1038 fps, consistent at 600 yds
    14.6 mil to 400 & 26.3mil to 600

    I tried 220 smk's but never got great groups with them, 1026 fps, 9.5 sd.
    Click to expand...
    Well damn, maybe I will try 190 SMKs first. That sounds like a decent load. My barrel is a 1:7 twist, so slightly faster and may work in my favor if I want to go up to 220s.
     
    I'm wondering if I may need to lighten the bullet since it doesn't have has much barrel to stabilize. I'm going to try a few more groups and then switch.
     
    I like the 208s better than the 220s. Shooting from a 10.5 inch barrel.
     
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    My (Sierra 2180) 220 RN load shot better than the 220 SMK's; 9.5gr h110, 7.8SD, 996 fps
    Didn't take these past 100 yds.

    All of my loads seem to tighten up a little bit after annealing the cases.
    Using Hornady 300 BO brass, 5th firing on cases

    All loads with 190 Sub-X didn't shoot well enough to take past 50 yds.
     
    As much as people hate on the idea of a 16” 300Blk barrel, I have to think that a 1:8T stabilizes a lot more in a 16” barrel vs. a 9” barrel. It isn’t something I would care about in an AR15. If I am shooting 1” groups at 50 yds with a 220 SMK, velocity of 1050fps with an SD of 5, thats solid. But for the idiot like me who wants to shoot 600 yards with subsonics, I miss the ragged one-hole groups at 50yds with the longer barrel.

    I am going to try some 190 SMKs and 208 ELDs as well. I am half tempted to have a 12.5” 1:7T barrel spun up to see if that helps. Should have done this build on an origin action!
     
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    X-Caliber has a 6 twist 30 cal. My smith has had pretty good luck with the blanks. I've had a couple AR barrels from them, and the were pretty decent. They also make a 5 twist in 30 cal.
     
    I tried out the 190gr HPBT today and didn’t have great results:

    2.30” COL
    10.7gr A1680
    Avg: 1020fps
    SD: 22.7fps
    ES: 80.9fps

    To no surprise group sizes were 2moa+ with vertical stringing.

    What scales are you all using? I’m thinking because of the small amount of powder the deviation of my powder loads is a larger percentage of the overall capacity and vary enough to make significant speed variations. I’m using a Chargemaster Supreme and am usually able to get SD of 15 and ES of 40 on my supersonic .300blk loads.
     
    NHPiper said:
    Wondering if a balance beam is the best option but I can seem to be able to get single digit SD
    Click to expand...
    Agreed, I've got a friend that has a beam scale, I'll give it a go with the same load and see if there is any improvement.
     
    You do you, but I wouldn't give a rats ass about the SD of a .300 blk.
    My concern would be, is it sub sonic or supersonic?
    You're likely not shooting 1000 yards with your 9 inch barreled rifle.
     
    It might help, but I can still get single digit SD with the chargemaster and N110. The type of powder it throws helps.
     
    fdkay said:
    You do you, but I wouldn't give a rats ass about the SD of a .300 blk.
    My concern would be, is it sub sonic or supersonic?
    You're likely not shooting 1000 yards with your 9 inch barreled rifle.
    Click to expand...
    The purpose of my build was to chase 500-600yds with subsonics specifically l. Basically to have a large scale 22LR.

    The single digit, and it really needs to like less than 5 for the SD, is because the ES with subs is so important to vertical stringing. Even changing with the gun is zeroed at 50yds vs 55yds means at 100yds your dope is already off by 0.5mils.

    You get away with a lot more, like higher SD, if you are shooting supersonics in any gun.
     
    NHPiper said:
    The purpose of my build was to chase 500-600yds with subsonics specifically l. Basically to have a large scale 22LR.

    The single digit, and it really needs to like less than 5 for the SD, is because the ES with subs is so important to vertical stringing. Even changing with the gun is zeroed at 50yds vs 55yds means at 100yds your dope is already off by 0.5mils.

    You get away with a lot more, like higher SD, if you are shooting supersonics in any gun.
    Click to expand...
    Ok, that wasn't clear to me. It is definitely an other than typical use.
    I'd look at your primers. They can have a significant impact on SD. I imagine with such a small powder charge they have an even more significant impact
     
    fdkay said:
    Ok, that wasn't clear to me. It is definitely an other than typical use.
    I'd look at your primers. They can have a significant impact on SD. I imagine with such a small powder charge they have an even more significant impact
    Click to expand...
    Worth trying magnum primers?

    My SD is pretty decent. I ordered parts today to try a 12.5” 1-7Twist AR upper. I figured that will tell me if the bullets just need more spin.

    Don’t get me wrong, a 1-8 9” barrel on an AR15 that could shoot subs into 1.5” groups at 50yds would be fine for most applications. Just not what I want to achieve.
     
    Not sure this helps accuracy. Back to the local Smith who cut it.
     

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    5.7gr of Hodgdon Longshot under a 190gr is quietest I've been able to make them. Swap the bullet to 200gr Makers and it takes 7.4gr to get them to 1050fps. These loads shoot the same velocity from my 16" Mcgowen and a 9" Black Collar Arms barrel. The 16" is just quieter. Significantly.
     
    fdkay said:
    You do you, but I wouldn't give a rats ass about the SD of a .300 blk.
    My concern would be, is it sub sonic or supersonic?
    You're likely not shooting 1000 yards with your 9 inch barreled rifle.
    Click to expand...
    I am, but I’m kind of a weirdo
     
    EggsBenny said:
    I tried out the 190gr HPBT today and didn’t have great results:

    2.30” COL
    10.7gr A1680
    Avg: 1020fps
    SD: 22.7fps
    ES: 80.9fps

    To no surprise group sizes were 2moa+ with vertical stringing.

    What scales are you all using? I’m thinking because of the small amount of powder the deviation of my powder loads is a larger percentage of the overall capacity and vary enough to make significant speed variations. I’m using a Chargemaster Supreme and am usually able to get SD of 15 and ES of 40 on my supersonic .300blk loads.
    Click to expand...
    When we tested subsonic 308's in prototype guns about 12 years ago you had to "back-stack" the powder to eliminate the vertical stringing. You load the round, tip the gun vertical on the toe of the butt and tap it a little on the ground, then slowly lower it horizontal into your prone position. Completely impractical but helped with the vertical. When I built my own 300BO bolt gun I wanted a powder that took up all the case capacity and then I discovered those loads were loud. 🤷

    Check out 338 Spectre. A 300gr Maker takes up all the case capacity and 8.2gr of Unique gets to 1050fps with 90+ % of the case capacity. It just needs something faster than 1:6.5". I'm still getting ever so slightly oblong holes at 100yds at 9000ft asl. Probably needs a 5 twist with those looong copper bullets.
     
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    N105 gave me the best accuracy, lowest SD with 190 Sub-X, 208 Amax and 240 SMK.
    N330 with 190 is the quietest load but it hates the freezing temperatures.
    Some have reported good results with HP38 as well
     
    JR1200W3 said:
    When we tested subsonic 308's in prototype guns about 12 years ago you had to "back-stack" the powder to eliminate the vertical stringing. You load the round, tip the gun vertical on the toe of the butt and tap it a little on the ground, then slowly lower it horizontal into your prone position. Completely impractical but helped with the vertical. When I built my own 300BO bolt gun I wanted a powder that took up all the case capacity and then I discovered those loads were loud. 🤷

    Check out 338 Spectre. A 300gr Maker takes up all the case capacity and 8.2gr of Unique gets to 1050fps with 90+ % of the case capacity. It just needs something faster than 1:6.5". I'm still getting ever so slightly oblong holes at 100yds at 9000ft asl. Probably needs a 5 twist with those looong copper bullets.
    Click to expand...
    Agreed but there as some powder combo that give you the perfect 100 powder burn/100 case fill in 300 BLK with quiet powders.
    240 SMK with N105
    175 SMK with N32c Tinstar
    135 SMK with Trailboss.

    Same with 450 Bushmaster
    300 XTP with Trailboss
    395 SubX with N32C
    500gr with Blue dot
     
    st1650 said:
    Agreed but there as some powder combo that give you the perfect 100 powder burn/100 case fill in 300 BLK with quiet powders.
    240 SMK with N105
    175 SMK with N32c Tinstar
    135 SMK with Trailboss.

    Same with 450 Bushmaster
    300 XTP with Trailboss
    395 SubX with N32C
    500gr with Blue dot
    Click to expand...
    Ahhh, the magical unicorn load, lol 😂

    I haven't tried VV powders in BO yet. Might do that.

    I've heard trailboss is extinct. True?
     
    JR1200W3 said:
    Ahhh, the magical unicorn load, lol 😂

    I haven't tried VV powders in BO yet. Might do that.

    I've heard trailboss is extinct. True?
    Click to expand...
    TB has been out of production since Covid I think 2021- 2022 but apparently it might possibly come back in 2024
     
    I'm not trying to down on 300BO. It's got a wide role to fill. But for long range subs I just don't think it's the best cartridge. It has too much case capacity, which it needs for supers. But for long range subs I would be looking for cartridges that are specific to subsonic use. I necked a 338 Spectre up to 375 and seated a 400gr Maker. I'm tempted to do 3 more and send them off to have a chambering reamer and a die reamer made. Just not sure what the optimum case capacity for that bullet is. The Spectre case seems about perfect for the 300gr, wondering if the bigger 400gr might need a bit more space for a slower powder.
    1000004434.jpg
     
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    So glad I bought the big jug of trailboss before all that nonsense. That thing may last me my lifetime.
     
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