If your bottle is leaking air only when connected to the rifle, this is most likely the culprit!
If your bottle is leaking air when it is not connected to the rifle, then the issue lies with the valve (see related products).

This seal can wear out if you swap bottle regularly. So some spares is always handy!

Tip: extend this o-ring's lifetime by applying a film of silicone grease.

Not sure if this is the part you need? Check the parts-list!