To develop that big flexor/pronator belly, you need to use wrist flexion. Really heavy wrist curls bother a lot of my lifters' wrists, so we usually go for sets of 8-plus and seek a pump rather than maximal strength.
Wrist rollers are better suited for heavy wrist flexion, and this version is especially challenging since the resistance increases as you roll.
The Special Forces community has some of the toughest fighting men in the world, but the military's training methods are antiquated. And CrossFit isn't the answer.