First we start with a warthog, give it a tachyon for small slot and advanced gatling turret for large. (this is just because it makes it blindingly obvious) Make 2 main weapon groups, the first for the tachyon only, the second for the gatling. Go out, target something, the indicator for auto-aim works, change to weapon group 2 (gatling), the indicator no longer changes color.

oh ya, i noticed that too
i had mines as my main weapon and gauss secondary for a run to 18 and there was no auto aim for the gauss