so, train the computer vision model for a gun and train again for a shotgun. Run the two detection models at the same time.
Your approach is the typical "but if you really want you can use an atomic bomb to kill mosquitoes" - yes, you could do that, but nobody is paying $1 mil/year in inference costs (+some expensively licensed software to wrap around that) when it can be done locally with a $300 GPU (+ some expensively licensed software to wrap around that)