To add to what
@eric32 and
@Ned Ludd say, Litz gives a value of 0.54 to the Rt/R metric that puts a numeric value of how 'gentle' or 'sharp' the shank to neck sections' junction is. To put that into context 1.0 = pure tangent ogive (jump tolerant / easy to tune), whilst the original Jimmy Knox / Walt Berger VLDs are rated 0.50. So this TMK is very close to classic VLD and has a marked secant form ogive. As
@eric32 says, if at first you don't succeed, then go through the VLD tuning process as set out in various places. The reason for its difference from its same weight MK stablemate in this regard is that the 1960s (short-range) design has an Rt/R measured at 0.90, close to tangent form. The later 175gn SMK is even better at an even 1.00, a true tangent ogive shape.
Note that every TMK seems to be different in this regard, so what applies to one is unlikely to be true for others. (For instance, I use the 160gn 0.284 TMK with a measured Rt/R value of 0.84 and found it to be exceptionally jump tolerant.)
The reason the 168gn TMK has such an apparently high BC compared to the 178gn Hornady HPBT Match is that the latter produces a lot of drag for a modern bullet design. This is shown in the value given to the 'form factor', 'form' being balliistician speak for 'shape'. Normal G type ballistics derive their results from a combination of sectional density, a mass to diameter ratio that is fixed for every bullet weight to calibre combination and the variable 'form factor' that compares the subject design's drag profile to that produced by the 'reference' G-whatever projectile. G7 model in this case. The 'reference projectile' always has a form factor value of 1.000 and as it is drag based, a lower value is better, higher is worse. The 168gn Sierra TMK's form factor is rated at an average 0.984 over the 3,000-1,500 fps in-flight velocity band. ie it produces 1.6% less drag than the G7 Reference model. The 178gn Hornady HPBT gets 1.043, ie it generates 4.3% more drag than the reference model. That's poor for a modern allegedly long-range design. (All values from
Ballistic Performance of Rifle Bullets 3rd edition by Bryan Litz.)
The more recent Hornady 178gn ELD-M is a different animal though with 0.970 which puts it close to the very low drag 168gn Berger Hybrid (0.966) - a very respectable value for a 308 bullet in this weight band. I'm a fan of the 168gn Hybrid. However, unlike
@Ned Ludd 's experience with the Hybrid, it wouldn't perform at all well for me in my former 308 F/TR rifle's chamber at normal length jumps (10-20 thou') but performed brilliantly when treated like a VLD and put into the lands. No doubt it'll therefore do well at long jump values too. Others I know who tried this bullet in British F/TR rifles came up with the same result.
I have a couple of hundred 168gn TMKs to try, but no 308 Win rifle anymore, so they'll eventually get a try-out in 300 RSAUM F-Open rig at somewhat higher MVs. They've seen a fair bit of use by factory rifle 308 Win shooters at long ranges at Bisley and there are encouraging reports on some of our forums, so if this is a pointer they work just fine for at least some shooters and rifles without over much tuning it seems.
