I'm not completely sure what you intended, because there may be something implied that is left out. Perhaps you're saying that gun laws can work indirectly, preventing mass shooters from obtaining guns in situations where the shooters themselves aren't buying the gun (for example, by taking it from a friend or relative). The difficulty here is that if you're going to truly challenge whether mass shooters can circumvent gun laws, you need more than a suggestion of how to fill one gap. Obviously this is difficult, but that's the nature of laws, and this challenge will probably have to mirror that. That said, the topic statement is pretty absolute, using "will" instead of "will often" and throwing in "easily", and, perhaps more than anything, including that not "any" gun laws would succeed. We could challenge it pretty well just by proposing that there is probably some kind of law that would at least make it difficult for some mass shooters to get guns. (Which is sort of what you're doing, but we still probably need more.) So, to expand your challenge, you should first address the more obvious target by making a separate premise, "Some new gun laws would make it hard for people who commit mass shootings to buy guns directly." and then modifying your existing premise, "Imposing severe punishments on those who enable illegal gun purchases would make it difficult for many people to obtain guns if they can't buy them directly." This actually kind of works without that much trouble, because we will have kind of avoided the details, but now those details can be filled in by writing supports for those two statements themselves. It is important to argue both of them to argue that new gun laws should be passed. (As an aside, this topic statement might be more useful to argue against if it wasn't absolute. It might be important to make it "hard" to circumvent the laws, rather than just "not easy". That said, making it "not easy" at least means that we may have prevented some shootings.)
The statement above appears in challenges, supports, or specifications of other statements.