I am at a loss on diagnosing my problem. I have loaded thousands of cast cowboy and other pistol loads on a Dillon 550 for years. Never jacketed or plated. I recently loaded some plated Berry's 9mm bullets into mixed brass. Some of the bullets could be pulled or twisted by hand. Increasing the crimp until the bullet held,yielded obviously over crimped rounds like the ones in the recent thread. I am using Dillon Dynamic dies. I called Dillon. He said that I had a resizing issue. He said that a seated bullet should hold, even before being crimped. This is not limited to a certain brand of brass and happens in 10 to 20 percent of loads. I can't find a measurement problem with any of the bullets or brass. What am I overlooking. For the record, some of the brass is Glock fired. First post here. Thanks fellas.