Within the armouring community there is an idea that is taking foothold. This idea is that some armour parts, such as helmets and breastplates, were not only raised from a uniform sheet of metal, but also, an maybe more commonly, by forging out a thicker plate to obtain the final product of desired thickness. This is backed by medieval visual sources and documents. Today I tried this technique myself, with the aid of a striker and a specially designed sledgehammer. I started forging from a round slab of steel, 13 mm thick, 17 cm in diameter and 1.7 kg heavy. I started alone, with a simple dishing hammer. After a while I realized that I would need a helper, as dishing 13mm of steel to 2 mm on my own, would be too big of a task.
So I forged a sledgehammer, 3 kilos heavy with the "beak" 23 cm long, to allow the striker to reach deep parts of a helmet/breastplate. The results were very encouraging. Having a second person helping with the hammering steeply increased the efficiency of the squashing technique, where the material while being forged on the surface of the anvil forces the surrounding material to move, thus making the whole plate deeper and deeper.
The inside of the would-be helm looks remarkably similar to some museum examples, where there are plenty of hammer marks, showing that those examples were indeed forged mainly from the inside, rather than raised from a flat sheet of steel from the very start.