My Formor Mage always carried a big ass revolver, because Gremlins.
An AR isn't that complex a machine, but there are still a lot of little springs and things in there...optics...electronics in any SR modern-era weapon.
Wheelguns don't have any blue smoke to let out.
Even without Gremlins, you probably don't have the skills to maintain a weapon like an HVAR (unless your mohawk is pinkish, and such considerations aren't an issue...in which case, carry on).
For more dakka, I'd say shotgun. Still simple, still big and effective. Spending a lot of hard-won cash on something like an HVAR leads to wanting to dump points into it, which is just throwing good resources after bad. You're a Mage. Any action you spend throwing lead means you're out of your lane, doing amateur work when there are professionals available.
You know why they geek the Mage first? Because they're more dangerous than a gun...you should always have one, but if you're using it, something's gone very wrong.
-Jn-
Ifriti Sophist