Modern body armor that can stop a rifle round is heavy, which raises the question of which body parts should be armored. For now, most soldiers wear a helmet and a vest with a chest plate, because those targets are vital — and because the head is often exposed, and the torso is the easiest place to carry a heavy load.
Another option is to use some kind of gun shield.
The Ajax gun shield attaches to now-standard side rails:
The Ajax consists of two spring-loaded frames extending on each side of the weapon, each holding one of the bulletproof side plates available for the U.S. Interceptor protective best. Each side plate weighs 1.6 kg (3.6 pounds) and is able to stop multiple hits by heavy (7.62mm) rifle bullets. When a bullet hits one of the plates the spring system absorbs much of the impact and returns the plate to its normal position that protects the face and shoulders of the user. Thus with Ajax the soldier can be looking for targets with his head and shoulders exposed. Without Ajax and despite the Kevlar helmet (also able to stop 7.62mm bullets) the face and shoulders (protected by some Kevlar, not bullet proof plates) left the soldiers exposed and likely to get hit if the enemy put out a heavy enough barrage of bullets. The face was also exposed to grenade and shell fragments. Ajax eliminates most of that vulnerability. Even most of the hands and arms that are still exposed now have some protection.
Ajax is meant mainly for troops on the defense (like guarding a base) or those on vehicles or boats. Troops manning light machine-guns or sniper rifles are particularly vulnerable because they generate the most effective firepower but can only do that it they show themselves so they can see targets and fire at them. Ajax is not really meant for troops out on foot patrols troops, because these soldiers tend to not carry the side plates. Being more mobile is a lifesaver and the side plates are but one of many items often left behind in order to reduce the load carried by the foot soldier.