I carry 3 on the belt and one in the gun. AR mags and a couple spare handgun mags live in the bag on my passenger seat, but typically when I take the rifle out I just grab a spare mag out of the bag and stick it in a cargo pocket. I also have some spares in molle pouches on my plate carrier, provided I have time to put that on.

Mag compatibility is more of a logistical issue to me than something I'm concerned about having available in a gunfight. I don't want to have to keep track of, maintain, or order a bunch of different types of mags/holseters/parts/etc. Commonality also allows me to have spares at the range in case something gets broken or somebody forgets something. If I had a "gun guy" officer like the OP who I knew was going to take responsibility for his own gear, I'd personally be fine with it, but our policy still wouldn't allow it.

There's a small part of me that wishes that I could go to a P226 RX as a duty gun, just because I'm a DA/SA afficionado and like the setup. I ran a kind of low-key range day last week, and brought my old P229 out just for fun. There was definitely a twinge of nostalgia shooting that thing on the PD range again, especially when it tore the X-ring out of a B8 demo-ing a drill. But as the instructor it would be sending a bad message for me to carry something other than what everybody else is issued, and I'm legitimiately confident in our P320s. They've been excellent duty guns.

there will always be an example to prove one's point.. as for the one where the one guy gave his mags to the other and he had not fired a shot... why the hell didn't he just give the gun over since it was obviously useless to him?