How to label inaccurate job titles on resume

I'm currently employed as a "web developer" for a company. That's what it says on my business card and that was the title on the job ad. The thing is I haven't done ANYTHING that can be considered "web development". The job is 100% system administration. There are several other people employed as "web developers" in this company. Only two do actual web development, the rest do many different things.

I'm planning on leaving the company and looking for a new job.

How should I label this job on my resume? I was employed as a "web developer", but a more accurate title is "systems administrator".

Since your new employers will ask your old employer what you did, the right way of saying this is:

Web Developer (System Administration responsibilities)

That way they know your title, and what you actually did.