Re: 1000 yard load for 23", 1-11" 308 barrel?
Posted: Thu Jul 27, 2017 11:51 am
With a 23-inch barrel 308, you simply cannot load anything to stay above transonic speeds at 1,000 yards in UK conditions. (Summer at 7,000 ft ASL in some semi-desert US location is very different and much more accommodating ballistically.) Moreover, it is difficult to practically impossible to keep 1,000 yard terminal velocities above the key 1,230-1,250 fps level where the US Army discovered a couple of generations ago in trials of the old FA Match 173gn FMJBT saw a near doubling of both wind drift and group dispersion compared to the same bullets at >1,250 fps. Those are ballistic facts of life and whilst it doesn't make it impossible to have a reasonable shoot on a largish target at this distance, means that no combination will achieve the precision capabilities that the rifle / ammunition is capable of at shorter distances - ie 800, maybe even 900 yards depending on the external ballistics.
So, what it's about is minimizing the damage. Some bullets perform better than others under these adverse conditions. The most likely outstanding one is that already mentioned - the 175gn Berger Tactical OTM which was designed by Bryan Litz for military sharpshooter / sniper use and to stay completely stable throughout both the transonic zone and as it falls through the sound barrier into subsonic speeds. It also has a very reasonable G7 'form-factor' of 1.018 despite being a short bullet designed for loading to the 2.810" SAAMI COAL and magazine feed in both bolt-actions and AR-10 type semi-autos. In fact as the 7.62 NATO COAL is shorter than the SAAMI 308 Win civilian round, it doesn't even get that in milspec loadings - 2.750" is the relevant figure. (By contrast, the venerable 175gn Sierra MK's form-factor is 1.085, so the Berger produces just under 2% more drag than the G7 'reference projectile'; the SMK 8 1/2 percent.) The Berger's downsides are twofold - finding any in our dealers; if you do fall lucky being charged over £56 / 100 the last time I looked. For optimal performance it is designed for 11.25-twist barrels and 2,600-2,700 fps MVs too, this twist rate having been adopted by the US forces as the standard for 'precision' 7.62mm rifles.
Others that have a good reputation in trans conditions are the original 155gn Sierra MK #2155 (a lot better than the higher BC but longer current Palma MK #2156). If you take the older SMK at the nominal MV of the NRA match ammo (~2,925 fps) and run it through the Applied Ballistics / Berger Bullets or JBM programs with its average 0.214 G7 BC (Litz) in last Saturday's Queen's finals 1,000 yard match - cold and wet - say 15-deg C, you get terminal velocities of 1,267 fps at 900 yards and 1,129 fps at 1,000 which means that some bullets from some individual barrels could have been subsonic at the target, more likely 99% were barely supersonic and in the worst part of the trans zone. So, it's fair to say the old 155 SMK still works, at least on a 2-MOA TR five-ring. (On the F-Class target with its 1-MOA 5-ring, only 25% of the area of the TR equivalent, the computed hit ratio drops dramatically even for a superb wind-reader.)
The old 190, 200, and 220gn SMKs were used by 'Match Rifle' competitors for many years and that's a 1,200 yard third stage discipline so anything they use works in trans / sub speeds. The 190 pushed at some pretty impressive MVs by a combination of long barrel / huge freebore and some pretty robust chamber pressures was an impressive performer until the current generation of very low form-factor / high-BC bullets appeared such as the 210gn SMK and Berger Hybrids.
The 175gn SMK as loaded in US 7.62 M118LR sniper ammunition is another very stable bullet in trans speeds. The problem is that with a 23-inch barrel, it is difficult to load it to MVs that will guarantee its remaining supersonic at 1,000 in all conditions. A situation where some are super and some are subsonic at the target gives desperately poor results - been there, done that with this bullet in early FTR days. With a 0.243 average G7 BC (Litz), you need 2,700 fps on a chilly Bisley day, and with say a 30 fps MV spread, some headwind gusts, bands of colder air blowing in etc, that really needs to be 2,750 fps average to be sure every one is still supersonic at this distance. A lovely day in the 80s F, and going up a bit above sea level as at Diggle and Blair Atholl means that 2,700 is probably fine.
The old 185gn Lapua D46 rebated boat-tail is a superb long-range performer - it should be as the Finnish D-series bullets were designed originally in the 1920s for consistent maximum range performance from 7.62X53R machineguns - out to 4,000 metres plus. It is still available and not too expensive. (This does not apply to the more modern 185gn Scenar - a fine bullet, but low BC and very much a short-range design.)
The worst types that one can choose are those short-range models with sharp boat-tail angles (the old 30-cal Amax series 208gn aside, 168gn Sierra, Speer, Nosler, Hornady HPBT match types); and VLDs. VLDs probably because of their length and sharp nose transitions are just as the name on the tin says - very low drag - when above trans velocities, but appear to be less stable and produce higher drag once well into it. John Carmichael (the HPS and Target Master ammunition man) did terminal chronograph tests at Bisley many, many years ago when Walt berger put his first 30-cal VLDs into production. They went faster than traditional tangent ogive bullets up to 800 yards for any given MV, but then slowed more rapidly thereafter and were going more slowly than non-VLD designs at 1,000. With what we know now though, that may have been partly down to shooting in a TR rifle - I can't remember the twist rate, but if it was 13 rather than 10 or 12 that would be a significant downside.
That is the other factor that affects using a factory rifle at extreme distances. The more work that Applied Ballistics does on extreme long-range bullet performance, the more valuable that considerable theoretical 'over stabilisation' appears to be in reducing the low levels of instability that appear in trans flight and increase drag. Litz is now talking about 8-twist as possibly being optimal in the extreme L-R role for 30-calibre bullets where dropping into sub speeds is happening. (It seems the military guys knew something we lost way back 100 years ago when they adopted 10-twists as the norm for their weapons.) A 12-twist 308 barrel in a factory rifle possibly (probably?) has a slight degrading effect on the bullet in the conditions we're discussing here beyond 800 yards - marginal ballistics from sub-optimal platforms for the distance.
So, what it's about is minimizing the damage. Some bullets perform better than others under these adverse conditions. The most likely outstanding one is that already mentioned - the 175gn Berger Tactical OTM which was designed by Bryan Litz for military sharpshooter / sniper use and to stay completely stable throughout both the transonic zone and as it falls through the sound barrier into subsonic speeds. It also has a very reasonable G7 'form-factor' of 1.018 despite being a short bullet designed for loading to the 2.810" SAAMI COAL and magazine feed in both bolt-actions and AR-10 type semi-autos. In fact as the 7.62 NATO COAL is shorter than the SAAMI 308 Win civilian round, it doesn't even get that in milspec loadings - 2.750" is the relevant figure. (By contrast, the venerable 175gn Sierra MK's form-factor is 1.085, so the Berger produces just under 2% more drag than the G7 'reference projectile'; the SMK 8 1/2 percent.) The Berger's downsides are twofold - finding any in our dealers; if you do fall lucky being charged over £56 / 100 the last time I looked. For optimal performance it is designed for 11.25-twist barrels and 2,600-2,700 fps MVs too, this twist rate having been adopted by the US forces as the standard for 'precision' 7.62mm rifles.
Others that have a good reputation in trans conditions are the original 155gn Sierra MK #2155 (a lot better than the higher BC but longer current Palma MK #2156). If you take the older SMK at the nominal MV of the NRA match ammo (~2,925 fps) and run it through the Applied Ballistics / Berger Bullets or JBM programs with its average 0.214 G7 BC (Litz) in last Saturday's Queen's finals 1,000 yard match - cold and wet - say 15-deg C, you get terminal velocities of 1,267 fps at 900 yards and 1,129 fps at 1,000 which means that some bullets from some individual barrels could have been subsonic at the target, more likely 99% were barely supersonic and in the worst part of the trans zone. So, it's fair to say the old 155 SMK still works, at least on a 2-MOA TR five-ring. (On the F-Class target with its 1-MOA 5-ring, only 25% of the area of the TR equivalent, the computed hit ratio drops dramatically even for a superb wind-reader.)
The old 190, 200, and 220gn SMKs were used by 'Match Rifle' competitors for many years and that's a 1,200 yard third stage discipline so anything they use works in trans / sub speeds. The 190 pushed at some pretty impressive MVs by a combination of long barrel / huge freebore and some pretty robust chamber pressures was an impressive performer until the current generation of very low form-factor / high-BC bullets appeared such as the 210gn SMK and Berger Hybrids.
The 175gn SMK as loaded in US 7.62 M118LR sniper ammunition is another very stable bullet in trans speeds. The problem is that with a 23-inch barrel, it is difficult to load it to MVs that will guarantee its remaining supersonic at 1,000 in all conditions. A situation where some are super and some are subsonic at the target gives desperately poor results - been there, done that with this bullet in early FTR days. With a 0.243 average G7 BC (Litz), you need 2,700 fps on a chilly Bisley day, and with say a 30 fps MV spread, some headwind gusts, bands of colder air blowing in etc, that really needs to be 2,750 fps average to be sure every one is still supersonic at this distance. A lovely day in the 80s F, and going up a bit above sea level as at Diggle and Blair Atholl means that 2,700 is probably fine.
The old 185gn Lapua D46 rebated boat-tail is a superb long-range performer - it should be as the Finnish D-series bullets were designed originally in the 1920s for consistent maximum range performance from 7.62X53R machineguns - out to 4,000 metres plus. It is still available and not too expensive. (This does not apply to the more modern 185gn Scenar - a fine bullet, but low BC and very much a short-range design.)
The worst types that one can choose are those short-range models with sharp boat-tail angles (the old 30-cal Amax series 208gn aside, 168gn Sierra, Speer, Nosler, Hornady HPBT match types); and VLDs. VLDs probably because of their length and sharp nose transitions are just as the name on the tin says - very low drag - when above trans velocities, but appear to be less stable and produce higher drag once well into it. John Carmichael (the HPS and Target Master ammunition man) did terminal chronograph tests at Bisley many, many years ago when Walt berger put his first 30-cal VLDs into production. They went faster than traditional tangent ogive bullets up to 800 yards for any given MV, but then slowed more rapidly thereafter and were going more slowly than non-VLD designs at 1,000. With what we know now though, that may have been partly down to shooting in a TR rifle - I can't remember the twist rate, but if it was 13 rather than 10 or 12 that would be a significant downside.
That is the other factor that affects using a factory rifle at extreme distances. The more work that Applied Ballistics does on extreme long-range bullet performance, the more valuable that considerable theoretical 'over stabilisation' appears to be in reducing the low levels of instability that appear in trans flight and increase drag. Litz is now talking about 8-twist as possibly being optimal in the extreme L-R role for 30-calibre bullets where dropping into sub speeds is happening. (It seems the military guys knew something we lost way back 100 years ago when they adopted 10-twists as the norm for their weapons.) A 12-twist 308 barrel in a factory rifle possibly (probably?) has a slight degrading effect on the bullet in the conditions we're discussing here beyond 800 yards - marginal ballistics from sub-optimal platforms for the distance.
