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Thread: Question, Murphy?

  1. #1
    Member Kurt S's Avatar
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    Default Question, Murphy?

    I've been reloading for my S&W M&P 15T, 5.56/.223.

    I was out trying several different loads yesterday, shooting 10 shot groups, two diff loads per session. I was picking up my brass and found this one;



    I can't say I've seen one like that and have no idea what caused it. The load was Varget, Sierra 55 or 60 gr bullet, once fired Winchester case, and under max by a couple grains. Any thoughts?

    Thanks

    Kurt

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    Mebbe, you wrinkled the case neck to the inside, when the bullet was seated, and shot it off along with the bullet.

    ?????

    I hope this holds you until Murphy comes along.

    Smitty of the North
    Walk Slow, and Drink a Lotta Water.
    Has it ever occurred to you, that Nothing ever occurs to God? Adrien Rodgers.
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  3. #3
    Member Kurt S's Avatar
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    Smitty, I'm very careful with my reloads and check em 6 ways to sunday and like to think I would have caught something like that. I don't think it would fit in the chamber if there was a wrinkle, but with the ramming speed of the action, who knows. It's not like it's shaved off either, it appears to be melted? I suppose a crack inside the case may be possible and torched it when it let off. Makes one wonder where that brass went, hopefully it vaporized and exited the barrel. Didn't seem to make any difference in subsequent firings.

    Anyones thoughts are welcome.

    Kurt

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    Hmmmmm!!! I've been looking at this over the afternoon and giving it some thought. Mostly I've been working on my Defender putting a new clutch master cylinder in it but I drop buy here to get some grease on the key board every once in a while.

    This case is erroded away by gas flowing past or most likely through a split or as Smitty said a crinkled case neck. Though both those rarely cause such a phenomenon. I don't think it matters which load or whether it was max or not, for some reason the gas cutting started. It would be my guess that a case neck was split and before the bullet left the case, the high pressure gas began to escape form the case neck through the split and past the bullet. It didn't come rearward because of a tight fitting shulder, I presume.

    If the case neck was not split, it split upon firing for some reason. Reasons for this would include, the case mouth was jammed into the throat of the chamber becasue it was too long or the case was crimped too much and/or with wrong type of crimp (roll crimp) causing a tight fit at the case mouth but not at the neck. Both of these would cause excess pressure and in an AR rifle would most likely blow the primer out the back. I see marks in the case neck that do not look normal and may be signs of high pressure. What does the primer look likeon this case?

    It seems quite apparent that it is severe gas cutting but that usually needs oxygen to do that much cutting. In a gas operated gun, after the bullet passes the gas port, gas pressure rushes the tube and acts upon the piston (in this AR) and moves it rearward which rotates the bolt head to unlock the action and the enertia of this small rearward movement and the trapped gas in the mechanism force the action open. This lets in a bunch of oxygen and stokes the flame hotter. (This is why the brass from an auto loader is ejected so hot.) Maybe this case was still flaming then???? I've heated cases with an acetalyne torch using the cutting head (bad idea) and when they get red and I hit them with the oxygen, they look just like this. (Don't try that at home.) It was just an experiment. I've never seen a case so cut by firing it in a chamber.
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  5. #5
    Member Kurt S's Avatar
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    Murphy, My first thoughts were "it looks like it got hit with a blow torch, oxygen added. I did some gas axe work on the pipeline, so I know exactly what you are saying in that regard,

    I don't use a crimp on these loads and check them all when done with a digital caliper for length. The chamber on this gun is for 5.56 and the loading data is for .223 so there should be a bit of cushion in the throat? Anyway, they are all sized to case, 1.760 and oal of 2.260 which should be ok and has been so far. I have used this particular load once before at 25 gr of varget, and kicked this one up to 25.5. max is something around 27.X so it should be ok. It was just this one out of 20 so I suspect it had as you and smitty point out, something to do with either the bullet or a defect in the neck. Second firing of a new case, and nothing jumped out while cleaning or loading it. I've had some cracks in case necks after a dozen loadings in my 7mm, but this one I hadn't seen. Here's a picture of the primer.


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