The people who've warned you off 168s really are your friends, at least so far as the Sierra MK model and those that are fairly close copies - the Hornady BTHP Match, Speer Gold Match, and Nosler Custom Competition - are concerned. They are superb short-range performers, but not only have relatively low BCs, are fundamentally unsuited for very long-distance use thanks to an over-steep boattail angle of around 13-degrees. The ideal angle is 7-9 degrees for long range bullets and going into double figures risks the airflow separating from the boat-tail walls and causing turbulence at transonic velocities leading to both a rapid loss of speed and instability. This also applies to the 180gn Sierra MK and to a lesser extent to all .30 cal Hornady A-Max models, the superb 208gn excepted. With .308 Win 168gn / 180gn bullet MVs, instability kicks in around the 900yd mark.
Berger's 168s are a very different matter and are all designed from the off as long-range bullets (unlike the others, the SMK launched 50 years ago as the 168gn International for 300 metre shooting - a role it was and still is superb in).
I agree with the advice given here by others, but you only hint at the shooting you're doing and that affects your choice. If you're sling shooting in a true Palma match run according to international Palma Federation rules, you're limited to .223 Rem / no bullet weight limitation or .308 Win loaded with a bullet of "less than 156gn weight" (hence the apparently strange choice of 155.5gn by Berger looking to squeeze the last ounce of performance out of the envelope). If run under local / US 'any bullet weight rules', you can load what you like and many shooters opt for heavier, higher BC models to reduce the effects of wind changes.
There are two potential downsides to this practice - the heavier the bullet, the heavier the recoil. The 175gn Berger BT Long-Range is a popular 'any bullet weight' choice as it gives substantially reduced wind drift compared to the 155s, but tolerable recoil in a 13lb sling gun. The 185gn Berger BT Long-Range is superb ballistically in .308W, but many shooters find the recoil too heavy with it.
The other argument against the heavies in this context is getting practice in wind reading. If you are going to shoot a lot of 'Palma Rules' matches, you may benefit from using the Palma bullets throughout to gain the experience and wind reading judgment with the 155s and avoid having to mentally switch between coping with two sets of bullet behaviour at long ranges.
If you're going to shoot F/TR, you go for the best ballistic and grouping combination that your rifle and bullet can deliver. Many F/TR competitors stick with the 155s because they do tend to group very consistently, but others have gone for 185s, and the 210gn VLDs. Some use both deciding which ammunition they'll shoot according to the conditions on the day. See the table below of 1,000yd wind drift in a 10mph true crosswind for a range of bullets at equivalent MVs (same muzzle energy).
The point behind all this is that as you've already discovered in your three-distance matches is that .308 Win is a very good 800yd cartridge but a marginal 1,000yd performer unless everything is optimised for this range - a bullet that groups well, has a suitably high BC and is shot at the highest velocity you can squeeze out of your rifle safely and consistent with good accuracy.
Incidentally, you are rather pessimistic in your belief as to what 1-12" twist will stabilise. The 190gn Sierra MK is a short bullet for its weight and this rifling twist works fine in all reasonable temperature and atmospheric conditions. You can also happily stabilise the 185gn Berger BT L-R, but the new 185gn Hybrid is a longer bullet and is marginal for this twist rate, really needing 1-11".
155gn (original Sierra Palma MK #2155) 3,000 fps 106.8" drift
155.5gn Berger 3,000 fps 91.1"
168gn Berger Hybrid 2,886 fps 82.0"
175gn Sierra MK 2,828 fps 97.1"
175gn Berger BT L-R 2,828 fps 85.8"
185gn Berger BT L-R 2,750 fps 81.4"
190gn Sierra MK 2,714 fps 90.0"
200gn Sierra MK 2,645 fps 86.0"
210gn Sierra MK 2,581 fps 77.5"
The other thing you are looking for is a retained velocity at 1,000yd that is not only still supersonic, but is as far above the speed of sound (1,122 fps under standard weather and pressure conditions) as possible to avoid transonic turbulence effects. The best of the 155s at 3,000 fps MV are running around 1,300 fps at this distance, the 185gn Berger at the equivalent 2,750 fps MV has another very useful 50 fps or so velocity. Your 175 and 190gn Sierra combinations are predicted to be travelling at 1,191 and 1,226 fps respectively under standard conditions, the 175gn value getting decidedly 'iffy' as strange things can happen as you approach the speed ofr sound under some conditions.
The worst pickle to be in is some bullets being supersonic and some subsonic as they approach the target and that's possibly what you were getting with your 175gn Sierra MK load. This bullet is VERY susceptible to this issue (been there, got that T shirt!); conversely, the 190gn SMK is famous for its transonic and subsonic stability, why it was long a favourite with British Commonwealth Match Rifle shooters who shoot .308W at 1,000-1.200 yards.
All data in the table is based on G7 BCs from Bryan Litz's book Applied Ballistics for Long Range Shooting 2nd ed and the PC ballistics program supplied with it. Using the G1 BC figures and tables / programs supplied by the bullet companies other than Berger Bullets are OK for shorter distances (up to 600yd), but can be seriously misleading (over-optimistic) for .308W at 1,000yd. Berger Bullets quotes both G1 and G7 BC for its longer range bullets on its website and also has a little free ballistics program that runs G7s for download.
Laurie,
York, England
