How do I remove the cap at the end of the magazine tube? Not the one that screws off, but the one that holds the spring in?

The spring retainer slides in with a friction fit.

Just stick a cleaning rod or a small screwdriver into the hole in the center and pull it out.

Watch out when you pop that retainer loose.  That spring is gonna fly out of there!!!!

Stick a screwdriver in and push it down about 2" then turn it a quarter turn in any direction and slowly release it.

This will work if you have a dimple on the underside of your mag tube. If you are sticking an extension, you should drill out the dimples with a 5/16" drill bit or do what I did and take the dremel and sand them down then clean out the whole tube from the metal particles.