What's stopping these guys from just going straight to bombings now? If the (ugh) cost/benefit of bombing is so much greater then they'd already be doing it.
They don't because it takes a lot more practical knowledge to pull something like that off, and procuring the ingredients necessary is much more likely to bring attention to authorities.
I just don't buy the idea that taking guns away from these nuts is going to lead to an increase in more deadly bombing attacks in the future. It hasn't happened in the UK, Australia, Canada, you know countries with similar political stability as ours who have enacted stricter gun control.
Pointing to examples of bombings in countries with vastly different political situations rife with sectarian violence doesn't really make sense.