Lucky:

I had the same question, and called a USERRA rep. (Can't remember where, but I found it on Google, and it was a government hotline). The answer is, even if you volunteer (not involuntarily called to extended duty), you are covered by USERRA. He showed me page and verse, on-line.

Now, some carriers enforce the 5-year rule more than others. If yours only gives 5 years, then I would approach it as a Personal Leave of absence, if you are starting to push your limit.