The aluminum tubes have a pin that blocks the tube and stops primers from falling on the floor and that pin works very well. Leave the pin in the tube.
What I do is pretty much what BigTex said. Somehow, figure your own way, get the primers on clean flat surface shiny side up NOT anvil side.
Pick up aluminum tube and press white plastic end onto a primer. Voila!! the primer has disappeared. Proceed to the next until all primers are in the tube. IIRC the tube holds about 120 primers.
Take the tube containing the primers and stick open end into the RCBS and don't force it. Remove pin. When priming a case the amount of force required is very little, we can thank Archimedes. Do not bear down on the handle.
If you did not properly assemble the RCBS you will know instantly. When you move the arm to set a primer in place the primer will jump out and land on the floor. This I know. It may happen so fast you don't see the primer jump so you may repeat the step several times before you are aware of the primers underfoot.
Get the hex wrench, unloose the set screw so that the shellholder jig can move freely. With the shellholder and primer cup in place lift the jig until the top of the primer cup is below the shellholder center hole mebbe .004 but you can eyeball it. Tighten the set screw while holding the jig in place. Yes, you need a third hand but I can do it by digit dexterity, therefor so can you.
Work the arm, a primer should land in the primer cup anvil side up and ready to be installed. If the primer is crooked your adjustment of the jig is no good, do again and raise jig higher but be sure the set screw is inside the adjustment ring on the jig.
The primer cup is composed of a rod into which is screwed the primer cup with spring. Strangely enough that w/spring assembly will loosen and begin to throw primers on the floor. I cannot stop it, it occurs with fewer than 100 primers so be prepared to remove the assembly, push against the spring, and tighten the screw.
If you are not paying attention and run out of primers and attempt to install a primer it is entirely possible the w/assembly unit will be embedded in the brass. This I know. You must fetch your hex wrench undo the set screw and attempt to remove the w/assembly. If you can't pull it apart get your favorite Universal decap rod, hammer, and gloves and knock the w/assembly free. Reinstall and get back to priming. After 4 or 5 of these events you will pay more attention. This I know.
Sounds like an adventure to Walmart but after a brief training period you will become quite proficient and complete your objective with no hand cramps.
