So if you're also a DMZ noob like me, wondering how to unlock the arrow bullet affix, I hope this might help (as I just discovered this today).
Click Gallery, followed by Catalog, select your gun, then select the third affix on the right.
If you buy and sell the SAME gun multiple times (I must have done this 30 times for the RAM7), you will increase your unlock progress.
Once you have maxed out the progress (16/16 for Ram), only then can you begin getting lucky with more advanced affixes.
I've included a screenshot of my mythic Sten – even though I've bought and sold it 5 times, my unlock progress is still 0/9.
Last two screenshots just shows where to go within gallery and catalog.