Any ideas on these neck marks?

Discussion in 'Reloading Forum (All Calibers)' started by oldduc, Jan 13, 2019.

  1. oldduc

    oldduc Silver $$ Contributor

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    After seeing a number of threads regarding donuts in the necks of brass, I decided decided to investigate some of mine. Using my Lyman bore scope I looked in the necks of a number of pieces of brass, including some once fired new Norma 6 Dasher that I recently acquired. The brass had been fired with Berger 105 Hybrid Targets, 34.8gr of RL-16, and FGM205M. Velocity average was 2970fps, and it is a very accurate load. Anyway, the inside of the neck didn't have any donuts, but they all had a series of radial grooves about 3/4 into the neck and arcing around about 1/2 the inside diameter. Also, the entire neck looks like the inside of the barrel, with rifling marks the full length of the neck. I checked some unfired brass from the same batch and they are shiny and smooth. I can't think of a logical explanation for either of these phenomena. Can anyone else? See photos below. One is of the radial grooves, and the other is too reference the space from the start of the shoulder.
    Gary
     

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  2. Dusty Stevens

    Dusty Stevens COVFEFE- Thread Derail Crew Gold $$ Contributor

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    Carbon scraping and pressure ring of the bullet. Remember youre pushing a pretty tight interference fit into almost diamonds. Brass doesnt stand a chance against that galling.
     
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  3. Texas10

    Texas10 Gold $$ Contributor

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    You had to buy a borescope, didn't you? :eek::p:p:p
     
  4. oldduc

    oldduc Silver $$ Contributor

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    That makes sense, they are grooves into the brass and can't be felt when pushing a bullet into the fired case. The bullet must chatter a little just before it stops. These are slightly compressed loads and so they slow/stop a little differently than a free moving bullet when seating them. This is a very accurate load, but I think I am going to change powders so the cases are not quite so full.
     
  5. oldduc

    oldduc Silver $$ Contributor

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    Hey, my life is too simple, I had to make it more interesting.....:)
     
  6. Zero333

    Zero333 Silver $$ Contributor

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    My assumption is those straight lines were caused by the expander button ( that's if you use one, and if the button is not the smooth type ).
     
  7. oldduc

    oldduc Silver $$ Contributor

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    I don’t use an expander, but these cases were not even sized anyway. I took them out of the box, trimmed the length to fit my chamber, chamfered/beveled, ran in a .242 carbide mandrel, and loaded them. I just looked at some that had the same treatment but haven’t been loaded yet and they had no marks in them. I don’t think seating a boat-tailed bullet could make them. I’ll have to investigate further when I load some more. It’s just a curiosity as it doesn’t seem to effect accuracy.
     
  8. fyrewall

    fyrewall

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    Spinning my wheels on this one - a wild guess is that the radial marks were caused by flowing brass subjected to some 60 thou psi (waves of plastic brass). If these rifling like grooves match the rifling they might indicate some type of gas cutting, again that 60 thou psi channeled in line with rifling features. I don't believe any carbon formed in the firing process is in the crystalline form like a diamond (hardness 10). The bullet, before being blown out of the case would act as sort of a dam with gas at 60 thou psi pushing at it and attempting to find some weak spot in the inside of the neck.

    Good work! I need a borescope.
     
    Last edited: Jan 14, 2019
  9. Texas10

    Texas10 Gold $$ Contributor

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    Questions: Regarding the circumferential grooves, are you shooting a tight neck chamber? Could these be related to brass flow?

    I see the same evenly spaced lengthwise grooves in some of my brass. Could these be artifacts of the forming process during manufacture that become visible once the brass has expanded and stretched a time or two?
     
  10. AlNyhus

    AlNyhus

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    You've seeing how 'round' the inside of your necks really are. ;) -Al
     
  11. oldduc

    oldduc Silver $$ Contributor

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    The gas pressure wave produced in the cartridge is traveling faster than the bullet, so I would think that it must bounce back when it is slowed by the base of the bullet. I am curious as to whether gas blowback can be "shaped" by the rifling. It's just something I've never seen before....but then, I never looked before.
     
    Last edited: Jan 14, 2019
  12. oldduc

    oldduc Silver $$ Contributor

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    It's a .272 (no turn) neck. After first firing I turn the necks just to make them round. This was the first firing for these, maybe they're just not round yet.

    I guess it could be from manufacture. I have no idea what Norma (or anyone else) uses to size the their case necks. I suppose they could use some sort of collet against a mandrel and the lines are artifacts from the collet segments pressing on the outside of the neck.
     
  13. oldduc

    oldduc Silver $$ Contributor

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    I think you're right about that.
     
  14. Dusty Stevens

    Dusty Stevens COVFEFE- Thread Derail Crew Gold $$ Contributor

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    You ever seen the perfect carbon patterns on the end of your barrel that match your rifling?
     
  15. bigstick6017555

    bigstick6017555 Silver $$ Contributor

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    what does the surface of the mandrel look like that you are using, i have seen galled mandrels make marks in case necks
     
  16. fyrewall

    fyrewall

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    A huge volume of gas is generated to push on the piston like bullet and everything else in all directions - my thoughts are that the 60 k psi exceeds the elastic limits of the brass neck surface inside the causing the radial marks, like plastic brass. Looking at black stuff (carbon) matching rifling at the muzzle is a good observation; the rifling type marks inside the neck appear to interrupt or cut through the radial marks. Would this be from a blast of 60 k psi gas following the rifling that was engraved on the bullet ? - that speedy shock wave preceding the passage of the bullet.

    To bad the bore scope could not capture the firing event in real time.
     
  17. oldduc

    oldduc Silver $$ Contributor

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    Yeah, they can do that. For some reason that same carbide mandrel picked up some hard crap right at the "ogive" and turned the neck of a freshly annealed 6BRA case inside out. However, I had polished it with 0000 steel wool before I did these Dasher cases and I checked some other new cases that I had run the mandrel in but not loaded and they were bright and shiny with no marks.
     
  18. oldduc

    oldduc Silver $$ Contributor

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    I'm not sure of the temperature of the gases created in a gun, but I had a plasma cutter that would make marks in anything. Think that's around 50,000 degrees though.

    Can't figure a way to get the bore scope to survive the firing event, but I once saw an internal combustion engine with a glass cylinder head and you could watch it fire. Maybe, ......nah, a rifle chamber probably has a little more pressure than that.
     
  19. oldduc

    oldduc Silver $$ Contributor

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    Yep, maybe that goes both ways.
     
  20. riflewoman

    riflewoman Gold $$ Contributor

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    Is the barrel cut rifled or button rifled?
     

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