I don't know I had the same problem with mine, I added this part in and it worked. Just take close look at step 7 of the mod and you'll find it. He doesn't tell us what kind of glue but I wanted to be able to take it apart again, thats why I used sticky tack.
You have to use plumbers putty or something like that, i used sticky tack to join the orange disks to the other part of the barrel. The part of the barrel that holds the darts.
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