I would be okay with limiting the sale of long guns (not just rifles) to those 21 or older, just the same as handguns.
High capacity magazines are generally items distinct from the firearm. They are often an after-market accessory. But even if magazines held 10 rounds or less, magazines can usually be swapped VERY quickly for a full magazine. In fact, high-capacity magazines are more likely to misfeed than low capacity magazines because of the pressure needed to feed the next round. So low capacity magazines are not really much of a deterrent to mass shooting events. The Uvalde gunman had multiple loaded magazines in his bag, ready to change out.
You likely know this, but some readers may not. A semi-automatic firearm is NOT a "machine gun" or necessarily a 'weapon of war.' It simply means that a after a trigger pull and discharge of the weapon, the recoil action of the firearm discharges the spent casing and allows a fresh round to enter the chamber. The operator of the firearm has to pull the trigger again to fire a round. There has a be a specific press and release of the trigger for EVERY round fired.
If I were ever in the unfortunate situation where I needed to kill or incapacitate everyone in an enclosed space, my weapon of choice would not be an AR-15 style rifle or any other type of rifle. It would be a 12-gauge pump shotgun loaded with buckshot. I can operate it quickly, take out multiple people with every round, and reload between each shot, if necessary. Shotguns were developed for trench warfare during World War I and have been used ever since for close quarters firefights by police, military, and civilians. Regarding revolvers, if I remember correctly, one has to be 21 to purchase a revolver in the United States.
