not backed by reality. not as a causal factor, and certainly not as a means to prevent mass shootings.
vast majority of "news-worthy" mass shootings happen with rifles that were already illegally possessed. you can't make them more illegal. the common refrain from some here is that cutting into legal ownership is bound to decrease the number of weapons and thus eventually amount of illegally operated weapons. it's not only a pipe dream, it implies actively infringing rights of people who obey the law.
it might instead be useful to look at why mass shootings of this nature happen more in past 1-2 decades than they did in say 1970s or earlier. rifles suited to this purpose have been around longer than we've been alive, and in huge numbers. it's not meaningfully easier to get them now than 50 years ago. but we observe more shootings of this particular nature now.