I have been reloading .357 mag for a couple of years now, without any serious problems. I was shooting my Phoenix comp targets last week, when I noticed the shot group increasing considerably. When I got home I checked the boxes and found I that on the worse scores I had used bullets from a new box I had bought.

I spent a couple of hours weighing all the bullets (500 !) and found that despite the box being labelled 158grain, the great majority of the bullets weighed between 178-180grain, an average error of 20+ grains! I won't say who the supplier was, because I have complained and they are contacting the company who cast the bullets. This may be a genuine one-off mistake, but it does seem quite a significant variance, partly because the powder I was using had no load data for such heavy bullets, so if these are going to be consistent, I may have to change powders. Some home cast loads that a friend makes for me only vary by about 0.5-1.0 grain at the most.

My question is: has anyone else found this much variance in commercial cast bullets? Also, would this difference in weight alone be responsible for nearly doubling my group size on a 25metre range?