Guns don't need to make sense -- in a free country people don't need to justify their actions. Rather, the government needs to be able to justify its policies -- we need to be able to show that the trouble and expense we are putting people to makes a real difference.
So gun owners feel punished when their guns are taken away, and they feel this punishment is more severe than all the lives that are taken away because of mass shootings?
The underlying thinking here is a kind of collective punishment trading or social bargaining; but that's not generally how regulating dangerous stuff works. There are lots of things that kill comparable numbers of people to guns. Do I need to list them? Generally with things that most people use safely but some people use in a way that gets them or others killed, we don't try to ban them but instead we:
(a) Criminalise misuse adjacent to death based on data (not based on emotion or intuition about what seems "dangerous" to uninformed people). For example, criminalising drunk driving because although it doesn't always kill people, it certainly kills them all out of proportion to other driving.
(b) Introduce restrictions in venue that are conducive to safety.
(c) Try to educate the public and post warnings.
(d) Opportunistically encourage people to delegate responsibility to make things safer, even if we don't change their rights overall. For example, Uber seems to reduce drunk driving deaths; but that was not accomplished by either taking people's cars away or preventing them from drinking.
With regards to guns, there simply is no data to support the idea that assault rifles are more deadly than other weapons; or that other things which leftists oppose -- open carry, 30 round magazines -- lead to much change in the overall rate of gun deaths. Banning these kind of things makes about as much sense as banning liquor that's "too strong" because you tried it and "no sane person would like it".
Rifles are rarely a cause of murder, suicide or accidental death. Yes, they are used in mass shootings -- they aren't used in much else. Do you know how rare mass shootings actually are, and how small a cause of death mass shootings actually are? If we took the same approach to other small causes of death as you are proposing taking with regards to assault rifles, what are the policies we might have?
What definition of mass shootings are you actually using? How frequently are assault rifles used in them? There is a lot of variation in the way they are counted so it's hard to talk about the real impact of what you're proposing.