Originally Posted by brando
That's actually a very important and overlooked issue - fitting the plates properly. I've seen a lot of guys wearing their gear with their plates sagging over their guts or just out of alignment. They're designed to be centered, otherwise you're missing out on some vital protection. Some rigs have really good plate retention, while others are essentially a pocket for a specific/common plate dimension. As suggested above, heavy duty velcro can help with that. Try sewing two strips of pile inside the pouch to attach a velcro strap across the bottom of the plate to hold it snug.
Exactly, Brando brings up a great point, people more knowledgeable than I in this have said that a good reference for sizing plates and proper placement is nipple to nipple and roughly 1/2 to 1 inch above the bottom of the clavicle notch (top of the breast bone to 1 to 1 and 1/2 inches above the belly button. While standing upright, the back plate should be situated the same as the front, as far as area covered.
The thinking is, you are providing as much protection as possible without impeding mobility as much. You can survive a gut shot much longer without medical treatment than you can a shot to the chest cavity in general (direct hit to the heart and or aortic artery being pretty much a DRT hit.
Basically if you can touch your chin to your chest while wearing the vest then your plate is too low and you need to raise it up so that your chin will rest on the plate instead.
Also that is a general guide, you want the biggest plate possible that doesn't impede mobility, for example, I run Medium ESAPIs in my IBA. It sucks as far as plate placement, way to low on me, however I'm thinking when I buy my own plates I'll run a medium up front and a large in the back, need to try it out on some issued plates before I drop the coin on my own though.