What do you mean by "government sector"?  Do you mean government jobs themselves or jobs that work for government contractors?

If you mean government jobs themselves, forget it.  The government does NOT pay the bucks for recruiters to feed them candidates.  And anyone who purports to give you any access to government jobs for a fee is a scam.  Move on.

No recruiters specialize in defense/government contractors per se.  Some specialize in aviation, some specialize in Quality Engineering - either an industry or a field.