Adler, it's more like a full power 10mm. I fitted a .400 Cor-Bon and .40 Super barrel in my customized Glock 21, so I can have 10 mm ballistics (.400 Cor-Bon) without buying a Glock 20.
In .40 Super, the Glock 21 is another animal. With certain loads it gets near .44 Magnum energy levels. Perfect for trekking into the woods.
The bottle neck cartridges work very well, and I have not had any reliability problems at all.