It’s not going to help. I’m sorry, I you seem to have missed the point. Pain and discomfort are things soldiers learn to deal with. That’s part of the training. A disciplined force won’t break because they’re unhappy from forced marches.
Also, they are trained not to grab the gun barrel, in this case we’re talking about accidents that can and do happen. In fact, I remember reading that one of the actors in Saving Private Ryan did exactly that during the mock boot camp they were put through. Prevent a soldier from feeling pain and you can turn a minor accidental injury into a casualty. That just doesn’t sound like a smart idea to me.
Again, you’ve got this one backwards. The ones you would suppress pain in are your shock troops. The combatants that serve as cannon fodder (literally.) You can use these guys in support of your more disciplined troops, but they’re the ones who are already acceptable losses.
In fact, this is something that actually happens. African Warlords are somewhat infamous for dosing their expendable troops with “bam-bam”, a mixture of cocaine and gunpowder, telling them the stuff makes them immortal, and sending them off to die. This is not something they do to troops they want to use again.