QSC (Quick-Change Spring System)

Rather than pulling the gearbox to access the spring guide, a QSC-equipped stock or receiver has an access point that lets the spring assembly slide out and a new one slide in directly, typically in a couple of minutes with basic tools.

The practical value is flexibility: a player travelling between a CQB site with a low FPS cap and an outdoor field with a higher one can rate-swap without a full teardown, rather than owning two separate guns or leaving the gearbox permanently set to the lower limit. It's increasingly common on higher-end sport and DMR-oriented rifles.

Not every stock or receiver supports one — it has to be designed in from the start — so a QSC is usually a feature to shop for when buying a new rifle rather than something retrofitted onto an existing one cheaply.