@Memoria for a RA mag I would recommend going pure R ATK, but if you do that you will need a second MAG if you plan on playing any melee classes.
13* weapons are the current highest. For weapon progression you should get to level 40 and run Advamced Quests. After you get 50 Photon boosters (you will get 50 very fast) you go to the Photon shop trader. They are located on the second floor of the Shop Area, the leftmost person at the counter. You can trade 50 boosters for a decent 11 star rifle.
If you can find a good group to run SH Advanced Quests after level 55, you will start amassing Unique Weapon Badges really fast. Also playing all Urgent Quests on Super Hard. Once you have 400, you can trade in your Unique Badges for a decent 13 star weapon if you haven't gotten a very lucky drop by then.
I used the Rising Badges from the Mission Pass to get the 12 star Nox Launcher and will be using my Unique Badges for a rifle.