***This has been fixed in update 17****
There is apparently a glitch that happens in the game where if you click last mission results you can potentially get all the master achievements for that specific slot (ex. Primary, secondary or melee).
In order to get this glitch to work you have to finish a mission and ONLY when the weapon levels up from 29 to 30 in the end mission screen will this glitch work.
When this happens usually you should get the proficiency achievement if you have not gotten it and the mastery achievement for that specific weapon.
So when you return to your ship go to the back and equip a new weapon in the same slot that you just leveled up to 30 with the previous weapon. Now click the menu button and navigate to the last mission results option. When the screen pops up both achievements should pop for that weapon you just equipped. Rinse and repeat for the others you are missing in that category.
If you level up the weapon during the mission instead of at the end of the results screen this glitch will not work.
PS. If your weapons is a 29 and really close to leveling up or about half way. Go to a void mission and do one of those because it usually gives you a good amount of experience to be able to hit that 30 when the match ends.
Leave any comments on how this helps you and please if voted negative let me know. Good luck
Thanks LSK ArchDeific for proving this works