Close Up Of An AR15 Barrel Post Proof Test

My wife’s coworker came across these photos and shared them with me. They are scanning electron microscope (SEM) images of an AR-15 barrel  The background that I got was that this sample is 4″ long from an AR15 barrel that split down the middle somewhat like Elmer Fudd in a Bugs Bunny cartoon. The company that sent this in was proof testing the barrel. The company that makes these barrels wanted to check the metal for an explanation as to why it split.

These next three images is progressively closer in magnification.




What is really interesting are these small horizontal lines in the rifling of the barrel. The following images were taken using backscatter electron detector where the contrast comes from the atomic weight of the material. So the brighter the pixel in the heavier the element in atomic weight.

Here is another image with less magnification. You can see how repetitive those lines are. They are only 0.5mm apart.  The large section running diagonally is a groove in the rifling and you can see the lands on either side.

In the photo above you can see what looks like a thick line of debris. Below is a close up image of that debris. I am not sure how this line of debris can form perfectly inline with the barrel.


In another section of the barrel there are these cavities that formed in the metal.

That nodule inside the crevice is typically an inclusion in the metal. My wife’s coworker was more interested in the darker gray mottling that is all around the crevice. Typically you would see some dark spots but not this prevalent, according to my wife’s coworker.


One problem is that this sample is very dirty. It has been contaminated with oil and too many people handling it with their bare hands. They probably should have cleaned the sample first before sending it. It is a compromise but if you want good analysis of the material, it needs to be clean.

Nicholas C

Co-Founder of KRISSTALK forums, an owner’s support group and all things KRISS Vector related. Nick found his passion through competitive shooting while living in NY. He participates in USPSA and 3Gun. He loves all things that shoots and flashlights. Really really bright flashlights.

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  • Graham2

    Interesting post.

    Horizontal lines are often seen in hammer forged barrels, so maybe that’s why they are present here.

    • SGT Fish

      my first guess was 3d printed, but it was purely a guess. Thanks for the info about the hor lines

      • Kyle

        The horizontal lines are reamer marks from taking the bore to it’s final size. In high end barrels these are lapped out before any rifling procedure. This is likely a button rifled barrel which just displaces the metal(as opposed to removing it) which is why the reamer marks appear on both the top of the lands and the bottom of the grooves.

        • MTGoat

          Beat me to it. Definitely reamer Marks.

        • FightFireJay

          Can’t be. All the markings are paralle or perpendicular to the bore, not to the twist/rifling!

          • Kyle

            I can say with 100% confidence that they are reamer marks. I manufactured barrels for almost three years and have looked down close to 10k barrels. The marks being perpendicular is exactly how you can tell they are reamer marks. In barrel manufacturing, the bore is drilled to roughly .005″-.010″ below its final diameter. A reamer is then run through the barrel. Imagine it as a really slow moving endmill being pulled through the barrel. Usually these reamers move at less than an inch a minute. The trailing edge of the reamer often leaves a slow radial spiral down the barrel. This is what is seen in the pictures.

            The way you can tell that it is likely a button rifled barrels is because the reamer marks are seen on both the top of the lands and the bottom of the grooves. During button rifling, the material is displaced, not removed, so any surface finish left after reaming will still be present. If this was single point cut or broach cut, the marks would only be on the top of the lands.

  • SGT Fish

    I keep trying to figure out what the outside profile of that barrel is. looks wild. maybe fluting of some sort?

    • Kyle

      It is a spiral fluted barrel.

  • Giolli Joker

    Is the “sample was dirty” comment coming from the lab?
    I’ve yet to see a lab that throws something in a SEM without sample preparation.

    • flyingburgers

      They can do low-vac or environmental SEM for cases where they don’t want to or can’t prepare the sample.

      Also the pics are missing their scale bars.

      • Giolli Joker

        Good point.
        Seems quite sloppy to me.

  • TheNotoriousIUD

    Probably because the barrel appears to be made out of wood…

  • 10x25mm

    Steel appears to have been continuously cast under conditions where a considerable amount of caster mold flux was ingested into the steel strand during casting. The ‘lines of debris’ are inclusions which were plastic at hot rolling temperature. Hot rolling stretches out lumpy ‘PART’ mold flux inclusions into linear strings parallel to the axis of the steel bar. Mold flux is intended to both lubricate the strand being cast and flux out hard reoxidation inclusions. The reoxidation inclusions (mostly aluminum and silicon oxides) are the hard ‘nodules’ you are seeing in the crevices.

    Amount and size of internal inclusions well above customary limits for ‘R’ suffix (rifle barrel) grade versions of AISI/SAE 4140 and AISI Type 416 steels. Split wood fractography of barrel typical when severe inclusions reduce the hoop strength & hoop elongation of tubular forms.

    Most likely causes: the steelmaker was running their caster too fast, or suffered a failure of their electromagnetic mold stirring unit, or both. Less likely cause: air aspiration at the joint between the tundish and the submerged nozzle feeding molten steel from the tundish into the caster mold.

    • Redfoot

      I understand about 1/4 of what you said, but I enjoyed it nonetheless.

    • Metallurgist? Thanks for the failure theory you developed from the available images. Sometimes I think I wish I had more metallurgy than the minimum requirements for my BSME.

    • RocketScientist

      Man, I don’t know how long you’ve been commenting here, but i just started noticing your comments in the past week or so. Been a breath of fresh air, There have been 2 or 3 posts where reading through the comments I’ve been groaning and debating whether or not to take the time to write out a response, then come across yours and thought “oh my, this guy beat me to it… and he’s RIGHT!”. Not sure your background (BS’s in Mech-E and Mat’l Sci, MS in Mech-E over here, working in the space/defense industry) but it’s always nice to see a dude who knows his stuff posting around here. I really appreciate the content you provide.

  • RickH

    Did I miss it? Who’s barrel, what company?

  • jon spencer

    What is missing is a slice of a barrel from the same batch as the failed barrel, that has not failed. Then have the SEM scan the same area at the same magnification for both samples.
    With the proof test exploded barrel only, we do not know what changed before and after the proof test.