After taking a look at the BC numbers using standard conditions at 1100 ft (28.35 BP, 55.1 deg F, 78% HUM), verses the standard sea level condition BC of .495, it jumps to a .5115 BC.
If you increase the BP to 28.53, it drops to a .5082 BC. But, when you raise the temp to just 80 deg F it jumps to a .5363 BC.
Raising the BP to 28.74 at 80 deg F = .5324 BC.
28.74 BP and raising temp to 100 deg F = .5576 BC.
Keeping temp at 100 deg F and lowering BP to 28.53 = .5615 BC
So, using standard sea level conditions (29.53 BP, 59 deg, 78% HUM) in a ballistics program, these corrected BC's (.532 - .562 BC) would be accurate, or the published .495 BC at your atmospheric conditions, same result.
Without knowing the exact temp and BP at the time of the test, and measuring actual drop on the target to 600 yards or so, it leaves too many variables that can poentially skew the BC.
I use an 8' sheet of plywood standing upright covered with freezer paper, using the top edge as a vertical hold point, then fire groups at each 100 yard increment to 600 yards recording "actual" drop in inches with no turret adjustments during the test. The rifle is zeroed "perfectly" at 100 yards before the test, or at least verified before hand. MV, temp and BP are recorded with the Kestrel
4000 at this time before each test.
I believe this is probably the single most effective, and accurate method to determine a bullets BC, but I also use the Oehler M43 with screens at the muzzle, and downrange screens hooked to the Oehler 35P for a two range MV determination of BC, or simply use the acoustic target and the M43.
Of the three ways of doing it though, I still prefer the plywood while measuring drop in inches over anything else. All have given me reliable numbers "if" all conditions are known exactly.
After BC and drops are known, shooting at these ranges again while "dialing" the zeros in will confirm your turrets accuracy, or point out a thread inconsistancy, uncalibrated pitch, etc, etc.
Twist rate will affect stability, and this will affect the BC, the other effects are probably unmeasurable, or unprovable, as the effects would be so small IMO. MV, temp and BP are major factors at LR, as is the recoil effect from various rests or shooting positions.
The 308 is an efficient round, as is the 300 WSM
, although I believe bullet inconsistancies and imbalances contribute to BC variations. Twist consistancy, bore and groove quality, and diameter, powder selection for a specific cartridge, load density, bullet base shape, jacket thickness, core hardness, muzzle pressure, barrel length and diameter are factors that contribute to bullet stability, balance, and integrity, and thus it's BC, all aside from the bullet itselfs consistancy.