Hi @the-real-droop ,
Agreeing with Kristof_2649, looks like you need a smaller lock ring, and one that is not floating. A floating lock ring is what provides all that play.
Both 2.1mm x 5.5mm and 2.5mm x 5.5mm are popular barrel sizes, you will need to verify which size you have. If you can fit a pen head inside of the plug hole then it is 2.5mm inner diameter (I.D.) This Post will provide further info.
If it is indeed a 2.5mm I.D. plug:
839-1010-ND might be a good 7/16 - 32 option.
1939-1880-ND | 1939-1889-ND would be my second guess although they do not provide thread details.