From an objective perspective, user's behavioral data on an electronic device would be a far stronger indicator of a potential attack than the purchase of firearms. I'm sure you're not saying that the government should have full access to behavioral data from every device/ISP.

A cowardly mass attack devoid of value for human life can occur using nearly anything (pressure cookers, fertilizer, vehicles, chemicals under your sink, etc, etc, etc). While certain individuals certainly should not be allowed to possess firearms (those with mental or criminal behavioral events/patterns - there is already a federal background check that occurs in obtaining firearms) it is near impossible to prevent someone from obtaining something they are dead set on obtaining unless you lock them up until their death.

Let's not forget that the only time the US has been invaded (technically not yet the US, I know) was when armed private civilians rose together and defeated the preeminent military power in the world. Had those people not been armed the US would not exist. Think how many potential conflicts on US soil the armed population may have deterred since then when you compare it's history with nearly any other country in the world over the last 250 years.