I often pump after putting a rigid metal ring at the base of my cock. This doesn't inhibit expansion at all, since arteries in your cock are deeper and under much higher pressure so a ring primarily compresse the veins, There are the "superficial" veins that you see just under the skin (and look so impressive), but more important is the deep dorsal vein on the top-side of your cock. It's this one that, if pinched, will maintain an erection indefinitely. It why some cock-rings have either a ball or a downward indentation that, if placed at the top, will apply pressure to the deep dorsal vein to maintain size after coming out of the tube. See the offerings at
BEST COCKRING FOR ERECTIONS - allknight.com (they seem to understand cock rings pretty well). This biggest problem with wearing a ring
during pumping is finding a size that's tight (small) enough to work and yet big enough to not be sucked past the tube's seal. Once the ring is inside, it's hard to remove the tube without dislodging the ring from it's best position - right at the base of your cock!