I'm hoping some can benefit from my little experiment/tinkering. I was having trouble with a load fitting in the shot cup, even though the wad was made for a particular payload of that size steel shot. I tried pushing the payload down with a spent rifle cartridge, but still couldn't get a good fit.
I was looking for something to use in my MEC Sizemaster press, that would compress the load. I took the inner and outer dimensions of the shot drop tube and the inner dimensions of the shot cup/wad with a mic. I was headed up to the rifle range to see if I could find a large caliber rifle/pistol case that I could put a dowel in. The dowel would fit in the shot drop tube and I would epoxy it into the spent cartridge case. The rifle range is 25mi away, so I stopped at the local builders' supply, with mic in hand.
I made this tamp/shot ram out of: 2.5-3" x 1/4" carriage bolt, w/head cut off, using the threaded shaft, 1-5/8" long; white, plastic, threaded "thingy" on the bottom (I'll find out what it's called), 0.62" diam.; 1/4" nut to jam the plastic "thingy" and keep it from coming unscrewed; 1/4" washer, for the shot tube to press down on; masking tape, 0.42" total thickness.
The shortness of the shaft allows the ram to come out easily, when removing the tamped shell; the tape is just tight enough in the tube, to prevent much wiggle/wobble; the shaft of the screw/bolt is wrapped with a little bit of masking tape, so the washer has a little play, but not much; the thick taped is not all the way down to the washer.. my mistake,, but it allows the washer to settle on the shell, without any torque to the side.. it could just as easily have been a little tighter, but I don't want to unwrap and rewrap the tape; the screw/bolt face sits flush with the white, plunger/tamp, giving a uniform press to the shot charge; the diameter of the tamp fits perfectly in the shot cup.
Here's the pic and the shell is a reloaded Herter's hull w/7/8oz #4, xxx-gr Steel, pressed with my Sizemaster. The hull comes with a 6pt... woulda preferred an 8pt.. oh well.
Take care, Ken