GBB mags? Then Hush is your answer!
Once these O-Rings are placed over your fill valve you will be able to silently fill your magazine without causing any gas spills. This is a renown inconvenience for some brands of GBB's, more specifically TM (but will work in competing brands).
And of course, they come in a charming yellow.
Contents includes:
5x Hush Fill Valve O-Rings
Will fit TM and most other fill valve designs.