How often are handguns even used in combat today? Personally I think that if there had to be a change in handgun ammunition I would suggest 10mm, they can use ammunition loaded down to .40 s&w energy for people who can't handle the recoil like the FBI did, plus if we ever have to fight a conventional military that actually uses body armor, you can swap out the barrel for a .224 boz (10mm necked down to .223) barrel.