You might use me for mass
Hint: Songs from a king
=> Psalter, pronounced "salter", another name for the book of Psalms (previous attempt: Song of Solomon (book of the Bible, King Solomon) or Psalms (Bible book of King David’s songs))
I could tell you your mass
Hint: Maker of the thing
=> Salter (previous attempt: Detecto? Healthometer? (well-known makers of physician’s scales) Salter and Seca start with S like the other answers but are not as well-known in my area)
I might even attack you
Hint: The team breaching
=> Assaulter* (previous attempt: SWAT (swat is to hit; SWAT team breaches entries)) * or there's this SWAT guy named Jason Salter
I sound like a cross
Hint: The land of the fling
=> Saltire, the cross in the flag of Scotland (previous attempt: (land of the St. Andrew’s Cross and the Highland Fling) or Saltire, the name for the cross)
What am I?
A psalter, a salter, assaulter, a saltire (previous attempt: I have no idea. Psalms Salter SWAT Saltire doesn’t get me anywhere but the letter S.)