I have to believe that people's perceptions have changed. Any competent lawyer should be able to explain to a jury that it is perfectly normal for someone to have the most common rifle in the country in their possession. After all, today someone is more likely to own an AR than a lever action cowboy rifle. Twenty years ago, it would have been easier to portray that person as an outlier.
That was not an issue in the study. There was no mention of the characteristics of the gun as being evil. The prosecution presentation did not bring such issues up. The gun was simply shown (as an image) to the study participants. Other research has shown that the more presence of a weapon influences juries. It would be foolish if the prosecution did not bring up the gun and simply presented it, for a defense lawyer to start talking about the gun characteristics. That might actually be counterproductive. One might say - oh, it is a most common gun. Prosecution - oh, the most common gun used in the rampages today.
Counter intuitively, some studies indicate that if the defense makes a big point about this or that, a juror might think that if they have such tantrum, so to speak, the initial point might be a good one or why are you complaining. OR - they only remember the initial negative and not your counter.
One might make a point before the jury presentation, that the prosecution should not be allowed to rant and rave with prejudicial statements. IIRC, in the case of the cop with the evil writings on his gun, the judge didn't allow that be used in court. Now, if the jury saw that in media - who is to know?
However, recall that the study did not make a point of the gun being evil - GET IT! It was just the image and the knowledge that the mock participants brought to the study.
Given the lawsuits running over AR advertising, the evil gun mantra is not dead in the legal world. Lawyers should be aware and the message isn't to go all nervous Nellie and only have bear spray. Just be aware of what is operative if you do end up in court.
Weapons, training, ammo types, competition have all been brought up to describe defendants as blood lusted. Just a fact of life. If you can afford it, mock jurors to test defense strategies (and for the prosecution) is a good idea.
Any competent lawyer is not a magic spell. It is an empirical question what works. Your lawyer will be competent only after the fact of your trial outcome.