Let's not forget that Donald Trump was a Democrat from 2001 to 2009. That wasn't too long ago! And, that would possibly explain his position on advocating the AW ban.
The bottom line on Trump is that he is an opportunist. If it will get him more power, more votes, more political capital by advocating for banning "high powered" rifles and "automatic" handguns, he wouldn't hesitate to come out swinging hard for gun bans. However, the one good thing about Trump is that he could probably care less about guns either way and truly would focus on the thing he might be good at - growing the economy. Whereas Hillary or Biden really would want to push an anti-gun agenda; Trump probably doesn't want to be bothered with it. Still, there are about 14 people I'd rather vote for in the primaries over Trump.