So, guns are clearly easier to use to do mass killings and that is why they are used. And, yes, I think if we were to somehow get rid of all the guns people would still use knives, swords, cars, and bombs to do mass killings. It does happen in the U.K. where guns are more or less unheard of. But, it does happen less often. I'm not sure if that is a simply a cultural difference or if it's because the ease of use of guns makes it more likely for people to commit a violent act. But, I do know that knife crime is a fairly major issue in the U.K. Although I think their murder rate is vastly lower than ours. But, again, I'm not sure that it isn't more of a cultural thing than an access to guns thing.
Anyway, bombs are really easy to make. I almost got killed by a pipe bomb when I was a junior in high school. Some 8th grader got ahold of a couple boxes of his dad's shotgun shells, broken them open, poured all the gunpower into a pipe, drilled a hole and put in a fuse. He put it in a bathroom at my high school, lit it and took off. I walked into that bathroom with a friend and it went off with my friend about 3 feet away and me like 6 feet away. My friend got hit in the arm with a piece of pipe but it was luckily a minor cut. I mostly just lost hearing for an hour. But, we were lucky because the kid stuffed socks into either end of the pipe. Had he been smart enough to put easily purchased metal caps on either end of the pre-threaded pipe, it would have exponentially increased the explosive force and likely killed both of us. So, if some 8th grader in the 80s can make a bomb with some pipe and dad's shotgun shells when there was no internet telling him how to do it, imagine what someone could do now?
On a side note, times have really changed. The kid who made that pipe bomb only got a week long suspension and was back in school after that. The Police were not called and he faced no criminal charges. If that happened now, that kid would probably be in jail for several years.