In "Bulletproof Water" (S03E10), the Armalite AR-50 makes it debut as Jamie and Adam use it to test whether water is bulletproof. In "MythBusters Revisited" (S03E14), the Armalite AR-50 is again used to retest whether someone flies backwards when shot. In "Curving Bullets" (S07E10), Adam and Jamie use an Armalite AR-50 to test whether a sonic boom from a gun will shatter glass. 

In "Mission Impossible Mask" (S09E01), Grant uses the Armalite AR-50 to test if you can start a merry-go-round spinning by shooting at it as shown in Shoot Em Up.
Jamie ar-50
Kari ar-50
Ar-50 table
Adam info ar-50
Grant ar-50
Grant info ar-50
Post fire ar-50