kelbro said:
Theory is great but in practice, hundreds of thousands of 175gr SMKs have been propelled out of 308 barrels with amazing accuracy at 1000yds in the 'trans-sonic zone'.
It depends on what you mean by 'amazing accuracy'. What I said there was that in an F/TR environment of a half-MOA 'V-Bull' and 1-MOA 'Bull', a load that gave around 70 fps higher MV than the M118LR did not perform well at 1,000 yards, in fact it performed rather poorly.
That was with a load that produced one of my smallest ever 100yd 5-shot groups with the .308 Win and which performed very well indeed to 800 yards in actual competition. 900 yards saw a falling-off and 1,000 saw a really big deterioration in the prevailing conditions. The latter point is crucial - a marginally ballistically adequate BC + velocity combination may perform extremely well in low atmospheric pressure conditions, 5,000 ft ASL in 100-deg F temperatures. This was the Scottish Highlands in temperatures that dropped to damn near freezing with a stiff headwind that strengthened in squalls. Put those conditions together and some bullets would likely be just supersonic, other just subsonic, but it was also obvious that wind changes affected this combination far more than the ballistic program predicted once the bullet became transonic. USMC snipers says the same thing about this bullet in the M118LR - 800 metres is the tipping point at that velocity.
Incidentally, the same 24-inch barrel rifle performed very much better at 1K using a 190gn Sierra MK load. That bullet has a longstanding reputation for happily sailing through the transonic zone and coping with dropping through the sound barrier. GB and the British Commonwealth countries have a form of prone rifle competition called 'Match Rifle' that is only shot at 1,000yd and up and is limited to .308 Win. Until the current generation of 210gn VLDs appeared, the 190 and 200gn SMKs were the norm because of their transonic speed good manners out to 1,200 yards (or more on some ranges in Australia).
The whole point about transonic speed bullet performance is that ballistic programs become less trustworthy, Once a bullet drops below 1.2-MACH, the pressure wave that had been riding ahead of the bullet clear of its nose moves back and that can cause turbulence. Because bullets have curves, from 1.2-MACH downwards you get supersonic air around some parts of the bullet body and subsonic around others. Some bullet cope fine with this, some don't at all and become unstable, some fall in between. The only way to find out is to actually try them out or rely on the experience of those who already have. In my case I found the 0.224" 80gn Sierra MK did fine at 1K even tho' it went through the target silently being subsonic and the butts crew missing its passage, the 0.308" 190gn SMK also performed fine, the 180gn was unstable, and the 175gn stable but grouping poorly.
