If there is gun control and/or greater restriction to purchasing firearms (e.g. not selling them at massive and easily accessible stores such as Walmart), there would be fewer guns purchased by law abiding citizens. Fewer guns owned by law abiding citizens means fewer guns available for non-law abiding citizens to steal.

This would make more sense to me if many black market guns used in shootings were legal in the first place

Automatic ARs have been illegal for years in the u.s. however they still have the deadliest impact when it comes to mass shootings. Since 2017, 12 of the 31 mass shootings involved assault rifles — which caused 39% of the deaths and 92% of the injuries

Yes, heavier gun control makes it harder for law abiding citizens to obtains guns, however, it would not do the same for non law abiding citizens, unless they are buying from those who are law abiding. The guns on the black market more than likely weren't obtained legally nor will they be sold as the such.