(Continued from Part 4.)
More on the 6.5 Creedmoor
In its development, many aspects of the 6.5 Creedmoor were combined to produce a very accurate cartridge. The 6.5×55 barrel twist rate is 7.8:1 and the twist rate for most 6.5CM rifles during its introduction was 8:1. This is ideal for the heavier 140 to 160 grain 6.5mm projectiles that give both of these 6.5 cartridges very desirable ballistic properties and fine accuracy. The long bearing surface of heavy-for-caliber bullets impart stability inside the barrel and outside the barrel as yaw rates are lower and the bullet settles down and could become more accurate as it travels out to long distances. In other words, if the cartridge shot 1 MOA at 100 yards one would expect that it would continue to hold 1 MOA at 400 yards and result in a 4-inch-diameter group. However it does happen on occasion that the round could become sub-MOA out past 300 yards as the bullet’s yaw rate decreases. (The stability actually increases, in flight.)
The effects of the wind is the factor that will most likely blow the bullet around and open up the group on distant targets. In the case of both 6.5×55 and 6.5CM, the difference in how much the wind can blow the bullet off course is cut in half between 400 yards and 300 yards. In a situation where there is a 10 mph crosswind the 143-grain ELD-X bullet that had a muzzle velocity of 2,700 fps would be deflected 9.7 inches at 400 yards, but only 5.3 inches at 300 yards. One might be able to shoot MOA out to a distance, yet the wind will likely open up the group significantly. This is why I set my long-range hunting shooting limit at 300 yards.Continue reading“Hunting in Desperate Times – Part 5, by Tunnel Rabbit”