With a 1:9 barrel you're pretty much at or edging past the stabilization limit with the 69gr round, the 77gr is definitely past the usually acceptable range.
30Cal said:I think it's laughable that they publish BC's to three digits; implying a high degree of accuracy in the number. Ha!
How much wind?mentioned mid-range to imply 600 yard HighPower shooting. I have recently developed a good loading for my Colt 1:9 upper. I am using the 69 grain SMK at 2842 avg fps. This gives me a .305 to .301 BC. Looking at the 77 grain SMK it shows .362 for its BC
At Camp Perry, one service rifle champ told me of the day he won the 1000 yard service rifle match with his M14. He had full right windage on his rear sight (either 33 MOA or 15.5 MOA) and he was aiming three target frames over to the right. That’s a lot of wind! Lucky he was not on the last target to the right, his aimpoint would have Canada!
I just did a check using the PCB freeware ballistic program using SMK 168 at 2560fps muzzle velocity, 600 yard zero, nominal BC 0.447.
The drop (drop of bullet from bore axis) nominally is 134.5 inches. Changing to BC=.440 gives a drop of 135.3 inches, and BC=.449 gives a drop of 134.3 inches.
135.3 - 134.3 = 1 inch. One inch out of 134 is a small enough difference that I'm not going to sweat it.
TimRB said:I used 75 degrees F, 0 feet altitude, 0 crosswind. These are the only environmental variables available.
BC = M/(d^2 * i) where M is the mass of the bullet in lb or kg, d is the diameter of the bullet in inches or meters and i is the drag coefficient or form factor.
A 168gr bullet weighs 0.0240 lbs and a 208gr bullet weighs 0.0297 lb and if you convert many common bullet weights to pounds, you'll realize that at least four decimal places is required to distinguish bullets that are close in weight.
Diameter in inches is typically given to three decimal places such as .233, .308, .357. Even if a caliber is listed to two decimal places, you'll notice that bullets are listed to three decimal places such as .452 for .45 Colt.
i is often listed with at least three decimal places in engineering literature.
_Bullet_ _BC_ _MV_ 0 200 400 600 800 | YARDS
0.305 2500 > 0.00 5.15 23.19 58.36 111.77 | wind (inches)
0.362 2500 > 0.00 4.26 18.80 46.75 90.41 | wind (inches)
0.305 2750 > 0.00 4.49 20.12 51.07 100.53 | wind (inches)
0.362 2750 > 0.00 3.72 16.33 40.67 79.76 | wind (inches)
0.305 3000 > 0.00 3.96 17.66 44.84 89.62 | wind (inches)
0.362 3000 > 0.00 3.28 14.36 35.65 70.26 | wind (inches)
0.305 2500 > -0.00 2.51 10.69 22.85 40.21 | drop (moa)
0.362 2500 > -0.00 2.38 9.89 20.39 34.75 | drop (moa)
0.305 2750 > -0.00 1.90 8.45 18.18 32.32 | drop (moa)
0.362 2750 > -0.00 1.81 7.83 16.25 27.79 | drop (moa)
0.305 3000 > -0.00 1.45 6.79 14.69 26.21 | drop (moa)
0.362 3000 > -0.00 1.37 6.29 13.16 22.53 | drop (moa)
Here is some interesting reading on BCs from Berger.
This article explains why Berger lowered their published BC.