The upper thoracic cavity is where the heart and lungs are. That’s what we’re trying to protect. And hard plates like ESAPI do a good job of protecting the front and rear of the upper thoracic cavity. The sides get more difficult, because you have arms. There’s still a lot of important blood vessels, and rather complicated joints in the area above and to the sides of where plates go, regardless of whether you are wearing an armor carrier like the IOTV or a simpler plate carrier rig.
The IOTV comes with a number of accessories to protect the neck, collarbone region, shoulders, and the sides of the upper thoracic cavity. These components are the yoke and collar assembly and the Deltoid protector. These consist of an inner soft armor component, an outer cordura casing, plus attaching hardware. In size L the total weight of these accessories is 3.87 lbs.
We can contrast that with a ballistic combat shirt. This is the usual modern style of combat shirt, with heavier material for the sleeves and upper torso and lighter material for the abdomen, that’s designed for (somewhat) more comfortable wear with body armor. In the BCS, the upper chest and shoulder region contains segmented soft armor panels, providing the same ballistic protection as the aforementioned yoke and collar assembly and the deltoid protector, but the total weight of a size L Ballistic Combat Shirt is only 3.2 lbs. This looks like about half a pound of weight savings, but remember, this includes the combat shirt. A modern-style combat shirt sans armor weighs about 0.9 lbs. So, for system weight, we’re looking at more like 1.4ish lbs. of weight savings. Not a lot, but every little bit helps.
Weight savings isn’t the only gain here. We’re removing a lot of bulk from the shoulder area, which is a big win in terms of how much it sucks to wear. Deltoid protectors get caught on things. They make narrow doorways, crawlspaces, and vehicle hatches more annoying to move through. Less bulk means you can move through these areas faster. The bulk also makes weapon manipulation more annoying. In testing, soldiers unanimously praised the new ballistic combat shirts for being less bulky and annoying. The loss of the various straps and buckles to attach all the above components is probably also a big hit.
https://soapbox.manywords.press/2017/09/16/ballistic-combat-shirt/