Updating this thread following some replies in the Facebook group, including some good info from Jacob himself.
In case anyone is interested, here is the testing data from a quick test I did last night-Jacob Wilcox was 100% correct.
Velocity’s were virtually identical, and the SD’s for the 0.357’s were actually better in both conventionally rifled guns (AP5 & VP9 Match RCM threaded barrel) by about 7 FPS less than the 0.355” projectiles. The P30L (polygonal rifling) was only slightly worse on the 0.357” lot, at 13.1 SD vs 7.5 SD on the 0.355”

August 14, 2026

P30L 147gr 0.357”
15 shots
Avg: 948.2 FPS

P30L 147gr 0.355”
10 shots
Avg: 948.0 FPS

VP9 Match 147gr 0.357”
15 shots
Avg: 887.9 FPS

VP9 Match 147gr 0.355”
10 shots
Avg: 898.7 FPS

AP5 147gr 0.357”
15 shots
Avg: 935.3 FPS

AP5 147gr 0.355”
20 shots
Avg: 930.8 FPS