When tumbling rock in the Rebel 17 tumbler do you include the weight of the barrel in the weight capacity or is it just the max weight of the contents?

The capacity listed for tumblers is for identification only. With rock tumblers, you always want to go by barrel fill level. You want to keep it around 2/3 to 3/4 full at all times. This means on your 3rd and final step you will have to add filler like Non Abrasive ceramic media.