Hershel now uses a metal form, a inch and a quarter stee rod with the last inch turned down to about 7/8" and cut flat on the one side that someone made for him. Two ring clamps. Dennis Glazener gave me one he made and it works just as well as a broom handle with ring clamps.
I am not sure Hershel really knows what gauge his sheet metal is, whatever is on hand. I like to use 16 or 14 gauge mild steel. You can get by with less. I like a bit more meat.
According to Hershel he has never watched his gun building videos. I have a short 59 min version of his 1980 video. Rooster crowing, audio up and down. Skips a lot of detail, got more out of his later ones but still if you want to learn his way take one of his classes. Have to call or write to get the schedule. Seems like they hold them when the mood takes them. Well worth the time, money and distance and the additional entertainment and feedbag you get to strap on is quite an experience.