What is less likely to explode/rupture in the unsuported area of a chamber?

Steel Cased ammo or Brass cased ammo?

Steel. Been tested by the US Military back in the 40's. 45ACP, and .30 USC were issued. Hatcher comments on how an unsupported case failure would not have occured had the pistol fired the then new steel case..( "Hatcher's Notebook",pg 183,3rd paragraph, refrencing softenss of cartridge cases and subsequent unsupported failures)

What are you trying to fire, unsupported where you expect the casing to do the work of the chamber?

Meplat-(Edited to add location of data in book)

Steel.