45.Can you explain to me how "chambering" an AR15 in a larger caliber adds weight? Or are you using AR15 and AR10 interchangeably, as if they are the same rifle? You can build a large portion of an AR15 out of composite's. Good luck doing that with your bolt action rifle.
larger calibre = heavier barrel. larger diameter + thicker wall. comparing 30-06 to .308 there obviously won't be much difference, but moving up from .223 to .308 and it is quite noticeable.
I mean you COULD make them the same weight, but then you'd either have a really heavy .223 barrel or a really light and potentially unsafe .308 barrel
for instance here's a good comparison with Bushmaster's MOE rifles. generally about 1 1/2 lbs more for .308 chambered rifles. Granted it's not only the barrel that's different, but overall the larger the calibre the heavier the weapon. pretty standard stuff.
http://www.bushmaster.com/firearms/MOE_series.asp