More leading yet better accuracy?
Here's what happened. I have an out-of-production 9mm that has terrible accuracy with 125 grn LRN Hardcast with the H&R profile (a raised band around the body of the bullet) but when it shot the bore was clean. Bullets had 1 band of hard yellow lube. Special Cleanup only involved picking away some wax/lube buildup under the extractor. Bullet size was .356 and my group size on paper would be 10 inches at 5 yards.
A couple weeks ago I tried a bullet from another company which used a messy blue lube. Same weight. Same hardness. Same diameter. Bullet shape however was a simple LRN, no shoulder or elevated band around the middle. Accuracy is much better, max group size is 4 inches at 5 yards. When I look down the bore, there are shiny lead slivers left inside.
I thought more leading typically indicated something was wrong and was associated with worse accuracy, not better.
What's going on here? Ideas?
Gun is a Bernardelli p-018 with round count <1000, bore not pitted, tight lockup, good slide to frame fit.