Most Lee pistol die sets will have a Powder Through Expander Die, and that usually has an aluminum "funnel" screwed into the top. That's so you can put a plastic funnel in there to dump your powder in from the little yellow scoop that comes with the dies. Remove that aluminum funnel, and the Auto Disc Powder Measure screws into that die. There's an expander plug in that die, when you raise the case up into the die the plug goes into the case, forms the case to the right ID, expands the case mouth a tad for seating the bullet, and the plug also pushes up on the tube on the bottom of the Auto Disc unit, actuating the powder measure and dropping the charge through the center of the expander plug.
These are the parts to a Powder Through Expander die. The aluminum funnel is the piece on the left. Remove that, then screw in the powder measure in its place.
This is an Expander Plug, the center is hollow for the powder to fall through. Right in front of the shoulder there's a tiny taper that bells/expands the case mouth, and the shoulder limits the amount of belling/expanding. When the case hits the shoulder, that pushes the plug up and operates the powder measure.
Rifle die sets don't have the expander plugs, but they operate the powder measure the same way. The Auto Disc doesn't have the capacity for large cases like .30-06 and such, you'll need to measure those charges with a scale and dump 'em by hand, or use some other kind of powder measure. If you get the Double Disc kit, you can do .223's OK.
Lee recently released their Auto Drum Powder Measure to take the place of the Auto Disc, and that solves that problem. It mounts and operates exactly the same way. You can also dial in powder charges more precisely with the Auto Drum. It's a modified version of their Perfect Powder Measure. Gotta get me one.
Some guys will buy an Auto Disc or Auto Drum measure for every set of dies and leave it mounted on there. I'm too cheap for that, and that won't work with my die storage method either. But getting a turret for every set of dies and leaving the dies mounted in the turret is outstanding, works very well, they're always setup and ready to go. I have a turret with a universal decapping die in it, one with a bullet puller die, and one with an RCBS powder measure in it. Extra turrets come in very handy.
