All of the torso plates are 16ga steel. The shoulders, gauntlets, and below-the-waist armor are all 22ga steel. I've always done either 22ga steel or 14ga aluminum, but decided to give the 16ga a chance on the bossman's recommendation during one of the Warmaster's Workshop livestreams.
Edit:
Just in case anyone checking the thread out has any delusions in regards to my talent, I'd like to share the pictures of my very first suit, the one I technically got approved in back in late '08, early '09, when Mandalorian Costuming was still mostly a lawless jungle.


As you can see, it was a fair bit of a mess, and I doubt it would warrant more than a glance from the App Team before getting kicked back with a veritable mountain of required changes

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Then, my most recent officially approved kit, from circa 2013-2014

Still a bit of a mess in areas, notably bagginess of flight suit and those first run vac-formed Legacy shoulders being too small, but leaps and bounds improved. That was after five solid years of practice, well over a dozen separate sets of torso plates hammered into existence, not counting utter failures and an ever-varying number of other armor parts. The proper armor build is all about time, practice and patience.