I wouldn't want him to go crazy with the gun use and become the red white and blue Punisher, no. I think the issue of him not using guns or weapons on the battle field is better described as impractical instead of unrealistic. I don't like when people complain about things being unrealistic in a comic book movie as well, but this isn't the same issue as "a green giant or norse God", it's more of "Oh hey, this mission calls for crazy far range combat, demolition and sniping." He's not going to be able to effectively use just his shield in such a situation. He's a soldier, he needs to be one at some points, but at others, he needs to be more than a soldier and rather a super hero. I don't want him always using guns, in fact, I'd go with his view on no killing in the modern day: things are different in a time of war, and right now there is no war. So once he's out of the war atmosphere he really shouldn't be using heavy weapons etc, but in WW2 I feel it would make sense.