Secondary explosion in Beowulf load

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  • ROLEXrifleman

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    Went to test out my 7” Beowulf SBR build the other day and also tested out my first batch of loads. I loaded 25 rounds but only fired 24. All 25 rounds were loaded with:
    Winchester WLP primers
    39 grains of H110
    Berry’s 350gr bullets
    Once fires AA brass from OEM ammo
    I was experimenting with COAL so 20 were seated to 2.200” and 5 seated at 2.155

    I loaded 3 rounds for my first magazine. 2 of the 2.200” and one of the 2.155”. All three shot with no issue. Picked up the brass and noticed no signs of overpressure. Figured all was good so me and my brother shot the remaining 21 rounds. At the end of the day I asked my brother if he felt any of the fireballs close to his face and he said no. I told him I thought I felt a couple right on me! I then reviewed some video he shot. The following are stills from that video:

    1) Right before the round goes off, all is good.



    2) And this is why we choose the .50


    3) Fireball starting to move forward

    4) Fireball is gone and now what appears to be the spent brass hitting the shell deflector can be seen. I circled it in red.

    5) The spent piece of brass is now engulfed by the beginning of a secondary explosion captured on the video. I used a red arrow to point to it in case it wasn’t obvious.

    6) Flame peaks. Red arrow again used to point it out.

    7) Residual flame can still be seen along with brass now flying forward.

    8) Last of the flame can be seen along with clear image of brass.

    9) The rear ends of the rounds fired showing primers.


    SO….. 24 rounds fired, my brother felt nothing and I swear I felt the flames 3 times. There were a total of 10 rounds caught on camera and this is the only occurrence of this happening. The first three rounds fired from the gun, as stated above, were two at 2.200” and one at 2.155” and I know for a fact I did not feel the fireball then and those 1[SUP]st[/SUP] three rounds were not any on the ones I have video of.

    And what are the forums’ thoughts about the secondary explosion? Powder issue, seating issue?
    Thanks in advance for any info you can provide.
     
    Last edited:

    oldpink

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    I'm far from an expert on these things, but it almost seems that your gun is starting the ejection cycle prematurely, possibly something to do with the way that the gas system or bolt was set up.
    Another possibility (mere speculation) is that it could be headspacing poorly, so do you see any signs of incipient case head separation that can sometimes (but not always) be a sign of out of spec headspace?
    Just looking at the case heads -- not the definitive way to judge pressure levels, but at least a good clue -- your loads don't show signs of excessive pressure.
     

    ROLEXrifleman

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    I'm far from an expert on these things, but it almost seems that your gun is starting the ejection cycle prematurely, possibly something to do with the way that the gas system or bolt was set up.
    Another possibility (mere speculation) is that it could be headspacing poorly, so do you see any signs of incipient case head separation that can sometimes (but not always) be a sign of out of spec headspace?
    Just looking at the case heads -- not the definitive way to judge pressure levels, but at least a good clue -- your loads don't show signs of excessive pressure.

    I studied the videos at great length and the 7 rounds fired by my brother all showed the bolt cycling as it should. Luckily the videos were shot on my iphone so the slow motion feature has been really helpful. In addition, based on the pics above, the bolt cycle doesn't happen till well after the bullet has left the barrel. The Beowulf is a very low pressure cartridge when compared to a 5.56, in addition the 7" barrel and pistol gas system make for a quick cycle.
    The bolt and barrel are a matched set coming straight from Liberty Barrels, the original barrel maker for AA arms. So I'm trusting they headspaced properly.
    I gave the dirty brass a once over and so no rings on the body of the cases to have me believe any seperation is taking place but I will be cleaningthem and inspecting a little closer both the inside and outside. I'll let you know what I find.

    And ya, from the looks of the primers I gotta a lot of room to work up this load, but that wont be happening anytime soon! lol
     

    Broom_jm

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    Too slow of a powder, and maybe too low of a charge. P.O. Ackley noted this, but it was in much larger, bottle-necked rounds.

    Since you're using a sizable amount of H110 powder, (a very slow-burning pistol powder) I would try switching to a magnum primer and working your loads back up from MIN. H110 requires both heat and pressure for the burn to complete; I suspect that is not happening in every one of your cartridges. (A chronograph would tell the tale, as the rounds where the powder isn't burning completely in the chamber would be much slower.)
     

    indyjoe

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    Too slow of a powder, and maybe too low of a charge. P.O. Ackley noted this, but it was in much larger, bottle-necked rounds.

    Since you're using a sizable amount of H110 powder, (a very slow-burning pistol powder) I would try switching to a magnum primer and working your loads back up from MIN. H110 requires both heat and pressure for the burn to complete; I suspect that is not happening in every one of your cartridges. (A chronograph would tell the tale, as the rounds where the powder isn't burning completely in the chamber would be much slower.)

    Or you will get very consistent chrono, showing that you are always spitting unburned powder out with everyone.
     

    rvb

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    And what are the forums’ thoughts about the secondary explosion? Powder issue, seating issue?
    Thanks in advance for any info you can provide.

    not an ammo issue, just the way ARs work.

    Gas (incl flame) comes down the tube and into the carrier. As the bolt unlocks (carrier moves to rear, bolt cams over) the rings clear the bleed ports in the side of the carrier. residual gas pressure (incl flame) exits those two ports.

    nothing to be concerned about. you just often don't see flame, but there's always high pressure gas bleeding off there. flame is from powder still burning off due to the short barrel and powder charge/selection/etc.

    could also be that there is still pressure/flame in the bore/chamber and the brass extracting is exposing it to the action area. Either way, not a secondary explosion, but residual gas / burnoff. A silencer would exaggerate this affect.

    that's a lot of powder to burn off in 7" of barrel.


    -rvb

    ps. slowing down the bolt unlocking might help some of this. less gas to the carrier, heavier buffer, etc. of course, that's secondary to it running right.
     
    Last edited:

    throttletony

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    not an ammo issue, just the way ARs work.

    Gas (incl flame) comes down the tube and into the carrier. As the bolt unlocks (carrier moves to rear, bolt cams over) the rings clear the bleed ports in the side of the carrier. residual gas pressure (incl flame) exits those two ports.

    nothing to be concerned about. you just often don't see flame, but there's always high pressure gas bleeding off there. flame is from powder still burning off due to the short barrel and powder charge/selection/etc.

    could also be that there is still pressure/flame in the bore/chamber and the brass extracting is exposing it to the action area. Either way, not a secondary explosion, but residual gas / burnoff. A silencer would exaggerate this affect.

    that's a lot of powder to burn off in 7" of barrel.


    -rvb

    ps. slowing down the bolt unlocking might help some of this. less gas to the carrier, heavier buffer, etc. of course, that's secondary to it running right.


    Based on this - faster powder, or less gas (adj. gas block) seem to be possible fixes.
    Also, I'd be interested to see if this happens with this same load in a 16" bbl.
    (I understand that the system works the same, but I wonder if you'd get a more complete burn earlier)
     

    indyjoe

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    Things get potentially dangerous if you develop a load for a short barrel and drop it into a longer barrel. It is possible that you go majorly over pressure, because you were blowing out powder before and now are fully burning it. Although with the slow powder, that is generally not an issue.
     

    oldpink

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    not an ammo issue, just the way ARs work.

    Gas (incl flame) comes down the tube and into the carrier. As the bolt unlocks (carrier moves to rear, bolt cams over) the rings clear the bleed ports in the side of the carrier. residual gas pressure (incl flame) exits those two ports.

    nothing to be concerned about. you just often don't see flame, but there's always high pressure gas bleeding off there. flame is from powder still burning off due to the short barrel and powder charge/selection/etc.

    could also be that there is still pressure/flame in the bore/chamber and the brass extracting is exposing it to the action area. Either way, not a secondary explosion, but residual gas / burnoff. A silencer would exaggerate this affect.

    that's a lot of powder to burn off in 7" of barrel.


    -rvb

    ps. slowing down the bolt unlocking might help some of this. less gas to the carrier, heavier buffer, etc. of course, that's secondary to it running right.

    Too bad rep no longer works, 'cause that would certainly have deserved some.
     
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