I was going to write this in the comments on the other solution but realized its much easier for everyone to see in a new solution.
September 4th, I unlocked this achievement by unlocking the muzzle and sight attachment, then BUY a shotgun for the medic class. It costs $3500 for the cheapest one. It cost the same for the rifleman gun but it requires 3 attachments. Go into your hide out and press up
on the directional pad. Then equip the 2 attachments with
and
, and then press up
again on the d-pad to bring the gun back down and it should unlock.
I'm not sure about any updates that may have caused this as I just started but I can confirm you need to BUY A GUN for it to unlock. I tried at least 50 times with the first weapon for rifleman and medic but it did not unlock the achievement. Hope this helps
Edit: Fixed Grammer and added buttons just because.