Its a 6-groove barrel.
I may have experienced an optical illusion when I first viewed the bore, before scrubbing away the brass buildup.
Here's a poor resolution image of the bore once the brass was scrubbed away:
![450b_polygonal.jpg](./download/file.php?id=1434&sid=0058346adc8e9801df783211db603656)
- 450b_polygonal.jpg (24.38 KiB) Viewed 12193 times
If you look at the two points where the slope appears to transition to the low areas, there's a thin darker line that may well be the shadow created where the high area drops down to the low area, AKA regular rifling. Before I scrubbed away the brass buildup and I could kick myself for not grabbing an image, albeit poor, of what I saw, the brass reflection was strong right where the high area transitioned into the apparent slope. That strong brass reflection continued down the apparent slope, stopping where it transitioned to the low area. It was a compelling sight to see in real time but my 63 years old eyes and the compromise image provided by the poor man's borescope, may have left me fooled.
You be the judge. I still intend to slug the bore to satisfy my curiosity.
Hoot