To what degree is 戈 used in common Mandarin?
It seems also that it means (or used to mean) "Dagger"/"Sword", however the only references I can find to this character says that that is an old, defunct usage.


戈 as a noun, simply mean 'dagger-axe or halberd'
The reason that it is an old, defunct usage, is no one are using it in modern warfare. Therefore the term 戈 as a weapon had disappeared from common vocabulary


  So, to what degree is this character used in common Mandarin? Is it just thought of as a radical, or does it have some other usage in speech?


戈 beside being a common radical, is also part of some 'warfare' related compound words and idiom.

Example:

干戈  = warfare

同室操戈 = infighting; fight between family members, partners or allies (
Two athletes from the same nation fighting for the Gold medal can be described as 同室操戈)


  止弋為武 
  
  
  toes + dagger-axe = advance with weapon = 武 (force); 
  止(stop)+ dagger-axe = stopping war = force
  
  
  #1 is the more likely origin of 武


戈 is often used as a transliteration word, for example: 戈壁 in 戈壁沙漠 is a transliteration of Gobi; 探戈 in 探戈舞 is the transliteration of Tango