When you select one of those two Media Types (Watercolor Paper - Radiant White, or Velvet Fine Art), the driver will automatically select the Rear Feed if the currently selected feed in the Auto Sheet Feed.
if you want to feed from the front, you have to go to the Paper settings and change the feed selection to Front Feed
However, you shouldn't have trouble feeding either HPR 310 or HFAP thru the rear feed.
But, the rear feed is a bit tricky to use. You have to put constant pressure (not lots of pressure, but consistent pressure) on the top of the paper as you slide it into the rear feed until the printer grabs it and pulls it in.
It took me a few tries at first when I first got my 3800, but after a few tries I got the hang of it and I haven't had a misfeed since.