Took it apart, the bucking is practically gone, only a small ring remains.
I went to the local Lowes to look for an oring or washer in the plumbing department that might work and found a couple to try.
One of the things was a valve seat about the same size as the bucking. I also picked up some 5/16" I.D., 3/8" O.D. orings.
I could get either on, but the rubber was too dense, didn't work.
I have a pack of +/-1/8" adhesive-backed foam rubber sheets I got a Wal-Mart or Michaels Arts & Crafts, so I cut out a piece and punched a hole in it to tightly slip over the end of the barrel, with the adhesive side towards the front end of the barrel, sticks to the flange on the barrel that mates to the gun shield.
Seems to be working fine

I fired a dozen BBs thru and each came out separately. I'll fool around with it some more to see if I get any misfires, but if it holds up that's one problem out of the way.