What's the average number of deaths per mass shooting, since 2000?
What's the average of a bombing incident?
It doesn't matter.
The fact is, in mass shootings, particularly school shooters and other grievance killers - including terrorists in the USA and also completely freak incidents like the Vegas shooter - people turn to semi-automatic rifles to kill, hence the focus on banning them, because they serve no useful, reasonable purpose in civilian life - there's no positive reason to have them. (I want them because I like guns, does not constitute a valid reason)
However, I assert that the same is still technically true of civvy street, assuming both bombs and guns are equally available. One person, one bomb, one bang, block of 120 flats, say 300-odd people, all done in a few seconds and easily planted. Same for a train bomb, or any number of other applications.
However, one person with one gun, relatively good marksmanship required, one casualty per hit, takes a fair bit of time, might even get spotted and stopped along the way. Odds far lower.
I think you don't understand how grievance killings work, especially school shootings.
In the vast majority of cases, somebody goes off the handle - they hastily decide to 'go out with a bang' due to how life has treated them, a good example would be Nicolas Cruz (the Stoneman Douglas shooting) it doesn't really require much planning, get your gun and your ammo and just go - armed with something like an AR-15, it's practically guaranteed to be successful.
Compare that to blowing up a building full of people, it has to be planned, someone with a cool head has to assemble a bomb, they need to learn the expertise before they assemble it, they have to get it there and plant it - they'll almost certainly need help from somewhere. Building bombs to do something like that is a seriously difficult thing to pull off, which is why grievance killers and terrorists, tend to default to AR-15s, rather than build bombs, where they're available, because it's easier.
Why do you think the ISIS terrorists who were building bombs, all had to go to various training camps, and get bomb making manuals from ISIS, along with help from other operatives? Because building effective bombs is complicated, difficult and very risky.