[dpvu]

Oils from inside the mechbox. The piston going forward in the cylinder is what propels the BB out. The piston head has an o-ring that seals against the cylinder and there is grease and lube to help keep things moving smoothly. If there's too much or it's too light (i.e. not viscous enough) small amounts can end up being pushed out with the air.

I'm not saying that's 100% what the problem is. I've never worked on a BOLT but it definitely sounds like a hop up issue and it is one of the sources of a potential problem.