One thing people overlook with military canteens and canteen holders-the fuzzy stuff inside is INSULATION. While in Cuba, we would chill our canteens, then soak the holders in cool water, and it would keep the contents cold most of the day. A wool sock would likely do the same over a bottle-we used to soak our berets as well, and that works great too. I dont think, side by side, there would really be any difference between an uninsulated plastic bottle vs an uninsulated metal one. Metal conducts heat better though, so it MAY be plastic works better-but, again, I think for the sake of argument, the difference is negligible.
I agree with Colorama too...tap water is fine here in the US. I refuse to purchase water when I get the same stuff for free. Save your money for something that ISNT given to you....
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my adventures