This is entirely plausible. Mass shootings aren't carried out by a random sampling of gun owners -- they're carried out by exactly the people who would be prohibited from owning guns if we tightened the laws around gun ownership a bit.
The best example of this is Seung-Hui Cho, aka the Virginia Tech shooter, who was able to buy handguns completely legally despite being involuntarily detained at a mental hospital less than two years beforehand. He then went on to kill 32 people.
You're inferring that mass shootings are carried out exclusively by the sort of person who would sell the government his weapon under a buy-back program. (Otherwise there still would have been shootings, even if fewer, with guns bought before the new regulations on sales) Just how realistic do you think that is?
Hmmm.