G20...still a glock
10mm baby...rocks like a 41 magnum, 15 rounds like a 9mm.
Why aren't these more popular?
You need big paws to grip the larger frame and you can't be recoil sensitive. Having carried a .357mag as a duty rig for years, I am not recoil shy in the .357mag or 10mm. Anybody who says that they are not .44mag. shy is lying; .44mag, yes recoil shy.
Second reason...expensive ammo to practice.

I have been shot by a 9mm, I won't carry one for protection. I recommend them to everyone who I don't like.

They don't work. I was not even shot in the kevlar. If I was shot by a 10mm, I wouldn't be here.

Now I am not talking about specialty rounds like the one's Colorado Springs PD uses...those will bust most 9mm slides....heck they even broke the PD's pistols until S&W fixed them with heavy springs. They are much closer to auto .357mag's in velocity and energy.

I'm digressing. Being able to shoot is much more important than bullet size. I remember an article I read in a combat defense course once, carry the largest caliber in the heaviest bullet you can handle accurately. For some it is a .22lr, for others it is a .44mag.

Don't just shoot at targets either, create stress shoots. Run a mile as fast as you can and try to shoot. This will duplicate your heart rate that has just been topped from adrenaline. These will best show you how you will shoot when you really need to.