Please help if you can:
I'm casting and powder coating for 9mm Luger. I'm using the 6 cavity Lee mold, .356 125 2r. I size the PC coated bullets on a .356 Lee sizing die. At the start of this process, the bullets were performing very well. I was using range scrap for raw material and I loaded the completed bullet to get around 1040 fps, easily making IDPA factor.
After shooting around 5000 bullets, I bought lead from a new source ... already mixed with tin and antimony. I think everything went OK with the first ones, but suddenly, at a recent tier 2 match, my bullets started tumbling. About two to five in every 100 shots. It seems more obvious at shorter ranges, which has me stumped.
After reading the posts I could find here on tumbling bullets, I stopped sizing and simply loaded the PC bullet. While the completed round now stand slightly proud of my case gauge, it does chamber. Yesterday I fired 25 shots of the larger diameter bullet. 1080 fps and almost all of them tumbled. Even at 25 yards.
The only difference I see in the bullet is a slight mark on the base. The first batch of lead seemed smoother.
It does have an effect on accuracy (rounds go low) and I'm not sure how to proceed. I'm shooting a big match on Saturday and, for the moment, I'm going back to sizing the bullet to .356.
I'm shooting a Glock 17 Gen 4. I clean the barrel after every session (around 125 to 150 shots). There is no leading that I can see.