Update:
I reslugged the bore and as I rotated the bullet through the calipers, the bore slugged out at .355 except for one part that was .356. (that might have been an error on my part though. it was only my second bore slugging)
different version of same mold: lee 124 grn 6 cavity mold that is dropping .358 at the base very consistently. The lube rings measure less. around .355.
lead: water quenched wheel weights. ingot BHN 14.
Lube: lee alox tumble lube. coated pretty well.
My most recent, and lowest power loads yet were:
2.9grn of titegroup (925fps on the chrono) - leading and 2 keyhole out of 20
3.4grn of w231 (950 fpson chrono) - leading and 3 KH out of 20
5.25grn HS-6 (1025 on the chrono) - leading and 3 KH out of 20
Leading was fairly evenly distributed along the whole length of the barrel. They all cycled the action, but I can't go much lighter and still retain function.
Thinking maybe I was using a light load under too hard a bullet and failing to get obduration, I tried 3.7grn titegroup with a 80% lead 20% wheel weight and got 10 out of 10 keyhole. 3.7grn using a 50/50 gave me 4 of 10 keyhole.
I tried some .356 purchased lead bullets with a hard lube ring (advertised BHN 15) that did not keyhole, nor lead as much, but still left noticeable leading after 20 rounds of each:
3.8 grn titegroup
4grn w231
5.6grn HS-6
5.5grn #5
most of the leading was back by the chamber.
I pulled a loaded round apart and it was still measuring .358 at the base. the lube rings are measuring less. around .355. Cartridge OAL is 1.08.
(I have a post over on castboolits similar to this one trying to widen the pool of brilliant ideas, so if you frequent both sites, yes this is familiar post
http://castboolits.gunloads.com/showthread.php?t=157586 they seem to think I am undersizing my bullets with my expander plug, but I tried their 38spl plug trick and it didn't really help any)