PUBLIC Forums => Public Forums => Call For Assistance Feedback => Topic started by: Shandra Lynch on September 13, 2022, 09:41:15 AM
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Good Morning All!
The 1st picture is from the breechface of an Inland M2 30 Carbine, and the 2nd is a cc. I don't really think that the BF marks are circular or arches. They are almost like a fan or pinwheel. What do you call these types of BF marks? Is there a category for them? Thanks much!
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Overlapping milling marks.
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Thanks much. I am looking for more of a descriptor, like granular, linear, circular, arches, etc. Is overlapping milling marks how you describe them in your notes?
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Yes that is how I document them in my notes.
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Ok. Thank you.
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Shandra, you would also not be incorrect if you called them arced BF marks, or overlapping arched BF marks.
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Thank you, Nancy
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I would sway away from arched as they are traditionally associated with Beretta/Taurus 92/96 series where there is clearly a one way arch from the face milling process. If someone told me "arched" I would think Beretta/Taurus 92/96 style.
While what you have shown does have arches, it is a result of a different process. Beretta/Taurus is a one time swipe in an upward motion. What you have is an endmill used in conjunction with a rotation of the bolt face. This is similar to the Ruger AR-15 bolt face (pictured below). Overlapping endmill marks as Mark mentioned.
As for a one or two word term to encompass the situation....that may take some time to figure out. If you want a description, I would classify it as a breechface produced by an undersized endmill to produce the case head pocket (or overlapping endmill marks).
Not sure if that was much help because I didn't come up with a term, but it gives you the jist of what you are seeing.(edited to say that overlapping endmill marks pretty much sums it up the more I think about it)
Regards,Chris
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Thanks, Chris.
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It would be curious to know what NIBIN is classifying this BF as? Maybe AFTE glossary committee can help define a standard term (ie. OE - overlap endmill)
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For NIBIN I'd likely say cross-hatched since you have overlapping marks that are not exactly perpendicular to each other but I think it describes the appearance generally.
CMC
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Good Morning All!
The 1st picture is from the breechface of an Inland M2 30 Carbine, and the 2nd is a cc. I don't really think that the BF marks are circular or arches. They are almost like a fan or pinwheel. What do you call these types of BF marks? Is there a category for them? Thanks much!
I like pinwheel or "swirl"
Message Bob Hart--he's really good at this. (And many of his terms appear in the Matrix texts).
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At first glance it reminds me of a spirograph
I like the name overlapping milling marks too. Seems to be the simplest term to call it what it is
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One more vote for overlapping arches over here.
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Chris and Mike,
Without using the machining method, what would your description be?
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If I were looking at the breechface on the firearm itself then I would call them overlapping milling marks as that is what they are and looking at the machined breechface this can be determined. If I were to only have the fired cartridge case and were looking at the breechface impression it would depend on the quality of the impression as to how I would describe them. In the case of describing the breechface impression only many of the suggestions could work - overlapping arches, cross hatching, overlapping circles etc. However if the impression was good enough I would not hesitate to state in my notes that they were consistent with being produced by overlapping milling marks.
Mike
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Christi,
I would use the machining method for 2 reasons.
1) There is no other way those marks would be produced during a manufacturing method.
2) We need to be more technical in our descriptions (when we know them) to align with other disciplines (outside of the forensic community). This aligns us with the machining community and their observations.
Chris
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Spiral sounds right to me, at least as a descriptor of appearance.
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Not sure if this has been answered in another thread, but do these BF marks have potential for subclass?
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Hi Raquel - these types of breechface marks would not have the potential for subclass carryover due to them being milling marks that overlap. The pattern created would be completely random...Chris's description explains why this would be - "...breechface produced by an undersized endmill to produce the case head pocket (or overlapping endmill marks)."
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Thanks Omar!!
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I would be cautious with subclass on these. If the breechfaces were hand milled then the potential for subclass would be eliminated as it would be virtually impossible to position these in the same way from breechface to breechface. However, I have seen the results from a CNC operation and the carryover was tremendous. In casework, it may not be possible to know if the overlapping arcs observed were from a hand or CNC milling process.
That being said, I would suggest focusing on marks produced from built-up edge, wear, chatter etc, but not just lining up the arcs or junctures from the overlapping arcs.
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I would agree with Nancy on the possibility of CNC subclass. Every CNC operation is extremely precise (typically to the thousandth of an inch or better) and it does the same thing every time. I see this on my CNC. This is an end mill operation where either the spindle (which spins the milling bit) moves in a circle to cut the breechface or the breechface is rotated relative to a fixed spindle. Either way the start point, feed rates and depths of cut are set by the program which could result in very similar toolmark patterns each time.
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looking at the picture though, there are plenty of unique features and breaks and that being an Inland M2, CNC would not have been around unless the government had a time machine :D