The big guns are hidden from sight anyway, and only brought out when need be. We don’t need any Oct 7th type attacks happening on home soil.
USA has 1-2 mass shootings everyday on average. This is far worse than a singular big attack. And how long would the reaction of police to any big attack even take? Is it actually realistic that they will have a useful impact with big guns?
2+ victims is a mass shooting per the FBI definition so while what you say is technically true it's also a particularly evil way to mislead the reader as the typical mass shooting of the FBI definition consists of 2-4 people shot over the course of an otherwise normal crime wheres the colloquial definition of "mass shooting" is more along the lines of a crazy suicidal person killing as many others as they can.