Counter-intuitive though it might seem, if you can't keep things above transsonic / supersonic speeds, a blunter / shorter (and therefore lower BC) bullet may give better performance and (assuming the sights have enough elevation to get there) give hits at a mile or so. Very long for calibre and very long-neck secant VLD type designs produce less drag and flatter trajectories than 'traditional' shorter / tangent shapes at higher speeds but are more likely to run into stability and turbulence issues as they drop down towards the speed of sound.
Lapua developed its 'D' series of rebated boat-tail FMJBTs way back in the 1920s to maximise the extreme effective range of 7.62X54R machineguns. During the final stages of WW1 massed long-range MG fire on fixed lines with whole machinegun battalions was widespread on the western front by both sides during nights to drop hails of bullets on supply areas, support trenches, road and rail heads used to bring supplies up and so on. Ranges as long as 4, even 5,000 metres were routine and after the war, a lot of development into ballistically optimal bullets was done with Lapua one of the leaders. By the time of WW2, massed MG fire had dropped out of favour, so ultimate range from rifle calibre bullets became unimportant -artillery shells though, now that was and still is a very different matter. As much of the flight is at subsonic speeds, transsonic and speed of sound barrier transition performance became very important as well as subsonic behaviours. By today's standards, the remaining D models such as the 0.308" 185gn D46 have really blunt shapes.
I learned this fact for myself when I started shooting 223 at modest MVs with 80gn bullets out to 1,000 yards 15 or so years ago. VLDs didn't do very well at this distance while the 'blunter' lower BC 80gn Sierra MK was a really well behaved number and did remarkably well - at least until the wind changed anyway. Definitely subsonic - the target markers always complained about the combination of a tiny hole and no sound signature. (In any event back then lots of people didn't believe the mouse gun would reach there never mind manage to occasionally produce a respectable score!) The original 155gn Palma 308 SMK (still around as product no. 2155 and loaded in the NRA's GGG match ammo) is the same - it barely stays supersonic fired from 308 Win TR rifles at 1,000 on lower lying ranges like Bisley, but continues to shoot well such is its inherent stability.
One thing to note is that many of the impressive US very long-range shoots you see on YouTube videos and suchlike on alfresco desert ranges take place in higher temperatures and altitudes than we'd ever shoot at here. Google 'Scotty and Chuck Long Range Challenge' YouTube videos for a whole series of ELR gong shoots with 6SLR and 6.5 Creedmoor rifles (and not super-length barrel custom match jobs either). Chuck's Creedmoor was shooting 123gn Scenars at 3,020 fps MV and I've been told by a participant that he got some hits (not on the videos AFAIK) at 1,900 and 2,000 yards, but they were subsonic at those distances. However, the 'range' is near Pyramid Lake in Nevada some 4,000 ft ASL and temperatures were in the 90s. That combination takes all the barriers out a lot further than we'd ever see in the UK due to the thinner air hence much reduced drag.
(I've always found the 123 Scenar an impressive performer out to 1,000 and doubling that distance is impressive from such a relatively light bullet. It also says that its form characteristics are such that it must cope very well with both speed transitions.) The 139 should be fine too and possibly even more so the antediluvian, but still excellent 142gn SMK in 6.5 Creedmoor, 260 Rem, and 6.5X55mm.
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