I've always had doubts about that being the bullet. Obviously I'm not an expert, I'm just a guy on the internet so don't take this as anything substantial as this is 100% armchair speculation and I'm fully willing to concede any part of this; but given what I know about the speed of a round fired from an AR-15 and how camera shutter speeds work, I'd assume that if a photographer was lucky enough with their timing like this, that the streak would be significantly longer. These rounds can go upwards of 3000 feet per second, so for the streak to be that small, you'd need an really fast shutter speed. But I don't think this was a particularly fast shutter at all, because there's significant motion blur elsewhere in the photo. The streak also seems to be slightly more in-focus than Trump does, but it's also possible that the photographer had the wrong focal length in the first place.
I dunno, I could be wrong and that could be the most legit photo in the world. It just feels very unlikely to me. It just seemed waaaay too convenient, on top of a rather long list of other individual conveniences that happened that day. I try not to think too much about it, because it's very easy to get conspiratorial about it.