308. But at what distance. I'm not questioning your skill in any way.
My point is that 10 years ago I'd have agreed with you. 12X is more than you will ever want to buy.
But people are shooting much longer distances now, mostly for fun. So a 24X scope makes sense if you are going to engage targets out to 1000 yards. After I took the sig class I went out and bought an 18X and am happy with it.
For all other uses, I don't need a more than a 3-9 Realistically. I can hunt deer at any distance with that scope that I'm confident I can ethically harvest that deer.
Though very very highly unlikely, If the shit hits the fan and I ever need to engage humans, 9x is enough to get you to the practical limits of the .223. Shoot, its not hard to ring the 12" gong at 200 yards with an Aimpoint. As an aside, I consider an Aimpoint to be inferior to good iron sights at distance. I can hold tighter groups at 200 and 300 yards with my CMP AR with open national match sights than I can with an aimpoint.
The other aspect is the desire to not just get a hit. But get a hit in the X. People are using 35x scopes to shoot F class. Which makes sense. In a controlled environment with solid support a higher magnification (If you have done everything else right and your fundamentals are solid) can help.