Topic: Is there a name for this type of breechface?

[Mike Beddow]

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

[Chris Monturo]

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

[Aaron Brudenell]

Spiral sounds right to me, at least as a descriptor of appearance.

[Raquel Pipkin]

Not sure if this has been answered in another thread, but do these BF marks have potential for subclass?

[Omar Felix]

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)."

[Raquel Pipkin]

Thanks Omar!!

[Nancy McCombs]

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.

[Mark Bennett]

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.   

[Mark Bennett]

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