I believe part of the reason for the weight restriction had to do w/ the old rules of using issued ammo provided by the host country... most of the NATO countries that used the .308 Win round use a bullet weight around 147-150gr or there abouts, or about 10g (a Lapua 155gr Scenar is marked as 10g on the box). Part of the HP fascination w/ super tight bores (.298x.3065) came from some of the issue ammo being rather undersized compared to commercial bullets, so in order to keep pressures and velocities up, shooters went to tighter bores. Somewhere along the line the 155gr SMK became somewhat of a standard, so the ultra-tight bores weren't strictly necessary but they held the minds of many for years and years. Now I think even the 155gr SMK 'standard' is no longer in force, as other bullets such as the 155gr Scenar is seeing a lot more interest, at least here stateside.
Another part of it is that when you start stretching the distance on a .308... you start running the edge of the envelope, so to speak, as far as bullet weight, B.C. and velocity. Some of the heavier bullets might have gonga B.C.'s, but at some point, a moderate B.C. and enough speed trumps a higher B.C. and a slower speed. Case in point would be a 190gr SMK moving out at 2600fps, vs. a 155gr SMK @ 2950... the 190 blows less in the wind, but not much... less than one click on the sight, actually, and it tends to wear out the barrel faster. A 175gr SMK @ 2750fps runs right about the same wind as a 155gr SMK... but change that to the 155gr Scenar, and you pick up about a minute worth of wind advantage. Obviously you can ram-rod the crap out of a 190 or a 175 and change the equation some, but a price has to be paid somewhere, either in barrel wear or recoil or both
The 155gr requirement came about by what would seem to be a rather capricious way, but ended up being little to no actual handicap in reality, at least not considering the cartridge restraint.
YMMV,
Monte
