Even on Sierras 175gr .30cal bullets have a BC of .505.
I know bullet shape, materials and construction all come into how a bullet will fly, but it would seem all of these things are summed up with one value which is the BC.
Yes barrel twist would come into it, but for some buying a new rifle, would it not be worthwhile getting one to shoot the lighter bullets?
Yes in general a heavier bullet will be better long distance, but its not the bullets weight alone, its that a heavier bullet generally will have a higher BC than a lighter one, but it doesn't automatically mean it'll have a higher BC.
I also plugged all the info into a JBM calculator and using the .505 BC Sierra 175gr compared to .504 BC Sierra 155gr even out to 2000yards the lighter bullet was better.