As said above, check the bbl hood, and check the ejection port for impact marks. Also check it for roughness or any type of roughness where it may be being hit by the ejected cases. This would be intermittent and some small differences between case types.
.3775 shouldn't be marking the BRY 124 bullet. As an experiment, size a case but do not flare the case mouth. Seat a bullet to the your OAL. Measure the case mouth right at the edge. This is a perfect size to taper crimp or maybe just .ooo5 more crimp. 😃.
You're responding well to a barrage of suggestions.
Below is how I measure case mouth and thickness. I hope I'm doing it right:
So I made a round with a seated bullet with no flare and no crimp. After, I pulled the bullet. There's some vertical marks from forcefully seating the bullet without flare, but no horizontal marks:
bullet diameter: 0.356"
case thickness: 0.0115"
case mouth diameter before resize: 0.3765"
case mouth diameter AFTER resize: 0.3735"
case mouth diameter with a seated bullet: 0.3785"
OAL: 1.1445"
Then, ran the same process with NO flare, but did crimp at the end. As you can see from the photo, this crimp left a witness mark:
bullet diameter: 0.356"
case thickness: 0.0115"
case mouth diameter before resize: 0.3760"
case mouth diameter AFTER resize: 0.3730"
case mouth diameter with a seated bullet: 0.3780"
case mouth after crimp: 0.3770"
OAL: 1.1430"
Same routine as the second round - ran the same process with NO flare, but did crimp at the end. As you can see from the photo, this crimp left a witness mark:
bullet diameter: 0.356"
case thickness: 0.0110"
case mouth diameter before resize: 0.3740"
case mouth diameter AFTER resize: 0.3735"
case mouth diameter with a seated bullet: 0.3770"
case mouth after crimp: 0.3765"
OAL: 1.1410"
On a side note, I did make a few dummy rounds that were resized, flared, seated, and crimped. There, the final OD of the mouth was 0.3785-0.3800", which also left horizontal witness marks similar to the severity shown in the pics above. I'm beginning to think any level of crimping with the SDB on plated bullets will leave a mark.