Yeah, the Lee Dies arn't top of the line, but we all don't drive Cadillacs either. The Lee dies I have work, as long as you don't expect too much and you set them up properly. With the collet dies, cartridge length can be a factor. Also, if you read the directions with the dies, you can increase neck tension slightly by screwing the die in a little further, 1/8 to 1/4 turn, and putting more pressure on the press handle. But be careful, you can go too far. Also, you mention different brands of bullet, check diameters, they may be more different than you think. Finally, yes, the sizing mandrel may be too large, the drill motor and sandpaper does work, but again, go slowly. If all that doesn't work, then call Lee. They supposedly have pretty decent cusomer support. Does this happen to be one of the delux sets that also includes a full length sizer die also?? If so try the FL die and see what it does. Just my opinions though.
bull