175's would work too.
I thought the objective was to get them to 1000 yards still super-sonic, not sub-sonic. I was always led to believe that the transition from super-sonic to sub-sonic destabilized the bullet.
There is what's desirable and what's possible. Fullbore / Palma rifles can keep their 155s supersonic at 1,000 in standard conditions from 30-32 inch barrels and MVs usually in the 3,000-3,050 fps bracket. The 155.5 Berger for instance is calculated to have a 1K terminal velocity of ~1,280 fps in standard conditions (59F / 29.92 inches mercury pressure). Ideally, speeds should attain or exceed 1.2 MACH or ~1,350 fps in standard conditions as trans-sonic turbulence occurs up to that point. US Army research decades ago showed that the crucial boundary is somewhere around 100 fps over the speed of sound - 1,215-1,225 fps is the key boundary and they meet that test.
F/TR and GB / British Commonwealth 'Match Rifle' with heavy loads in small primer brass and heavy, very low drag bullets such as the Berger 200.20X attain considerably higher terminal speeds at 1,000 and MR competitors shoot the 308 to 1,200 in this discipline, in excess of 1,500 yards on one Australian range.
For an 18-inch 1:14 twist barrel 308, this sort of performance is a pipe dream and the usual short-barrel / long-range policy of adopting heavier, high BC bullet loads using say the 185gn Berger Juggernaut is ruled out by the slow twist. Here in fact, only the older and shorter (hence blunter and higher drag) 155s can be used.
If you know you're going to be trans or even subsonic, the trick is to use a bullet that handles the two transitions well. The older 155 and the 175 or 190 Sierra MKs, the Berger 155.5 BT Fullbore and 185gn LRBT Juggernaut and some others fall into this category. You get larger groups and more wind drift than at faster speeds but they'll still work and even work quite well. Some designs cannot handle this, in particular the older short-range 168gn 30s such as the 168 SMK, Hornady HPBT, Nosler Custom CC. Much of this is to do with an over-steep boat-tail angle.
Then there are bullets specifically meant to cope with these circumstances. A good example is the Berger 175gn Tactical OTM which Bryan Litz designed for military type semi-auto 'Sharpshooter' or SDM type rifles with 18-20 inch barrels and standard military loads giving around 2,600 fps. Bryan shot 1-MOA 1,000 yard groups with these bullets in testing from a Larue semi-auto big AR.
There is nothing new either in 30-cal bullets achieving exceptional ranges irrespective of terminal speeds. The high spot was the 1920s when with WW1 experience behind them, national armouries developed L-R ammunition for machine-gun use. Shooting at out to 4,000+ metres was common by both sides on the Western Front in 1917-18 with the creation of massed MG battalions acting as rifle calibre lightweight artillery on the other side's rear areas and communications trenches and dropping literally heavy showers of plunging subsonic bullets on troops and supplies moving up to the front-lines during the night. The US contribution to this was the .30-06 M1 ball round using a 174gn 9-deg boat-tail FMJ bullet at a nominal 2,647 fps MV (nothing to do with the US M1 'Garand' rifle). It was introduced in 1926 and had a claimed effective range of 5,500 yards in machine-guns, most of which flight was subsonic. The famous Lapua D-series rebated boat-tail bullets which still survive in a couple of current versions were designed by the Finns between the wars for the same purpose and alongside the Swiss GP11 cartridge's bullet were arguably the best. (Ultra L-R use of MGs went out of fashion by WW2 and a common rifle and MG round was seen as preferable by most powers. The USA dropped its M1 just before WW2 and replaced it with the 152gn M2 30-06 ball as the heavy round kicked like a mule in the 1903 Springfield and its ballistics exceeded National Guard range danger areas.)
There are no 0.308" 175s that I can think of that are fully stabilised by the 308 Win in a 1:14 twist. Some are marginally so and will apparently shoot well at short ranges, but performance degrades with distance and there is also no margin to cope with ballistically sub-optimal ambient conditions such as very cold air in winter conditions.
