The bigger problem in trying to answer your question is that a lot of guys (and gals) carry thier own gear becuase they like it better than the issue stuff.
Where did military Camelbaks come from? Becuase a lot of units had purchased them with thier unit money, and everyone in a unit that didn't buy them was buying them anyway. Eventually, the military broke down and issued them rather than working on a small mountain of plastic canteens that has been growing since Vietnam. Everyone was buying reflex sights out of pocket for long time, now DoD is issuing them. The Army and Corps are FINALLY issuing goggles that work with helmets, after two decades of issuing tanker goggles to everyone that didn't even really work well with the helmets worn by armour crews, much less the standard k-pot, during which time people were buying them. The list goes on and on like that.
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-IronRaven
When a man dare not speak without malice for fear of giving insult, that is when truth starts to die. Truth is the truest freedom.