There's been studies that demonstrate there is no demonstrable safety difference between the use of slug-only shotguns, straight-walled centerfire (rifle or pistol), muzzleloaders (with or without Sabots) and necked centerfire.
The restriction in the SRAs to shotgun/ml only is based on perception & optics, not fact.
That said, I don't see a change to the permitted arms in the SRAs happening anytime soon...
Inertia is the name of the game when it comes to lawful devices in this state.
Edit: I also doubt the restriction is born of resource concerns, they have a hard time as it is hitting harvest targets in the SRAs on a consistent basis.