Sorry, just noticed it was a system db you were talking about. Same theory applies, try to shrink before changing recovery model.
See this article possibly:
How can IT departments tackle the challenges of a Big Data world? This white paper provides a roadmap to success and helps companies ensure that all their data is safe and secure, no matter if it resides on-premise with physical or virtual machines or in the cloud.
Having an SQL database can be a big investment for a small company. Hardware, setup and of course, the price of software all add up to a big bill that some companies may not be able to absorb. Luckily, there is a free version SQL Express, but does …
Using examples as well as descriptions, and references to Books Online, show the different Recovery Models available in SQL Server and explain, as well as show how full, differential and transaction log backups are performed