How to open a Crossbow? (Call of Duty: Warzone)

Good day. Condition is to kill 25 people from the infantry rifle using a mirror sight. I've leveled almost entirely one infantry rifle along the way, putting on all the open sights, then put the sights in the prefix which is written the word mirror BUT the crossbow is still closed and in a written of 0 of the 25 killed. What I'm doing wrong any schools the Google translation either need to use a certain rifle, or free it does not to but then why it did show in the list. Thanks in advance)