308 Ballistics Question

[Primer]

Shooting an FLCP-K, with 168gr., BTHP, Hornady Match Ammo. Ran ballistics using 2700 fps, .45 BC, zero at 100yds. Using Hornady and JBM programs, both indicated come up of 11.2 MOA. Made the adjustment and put three shots 12-13" high. Adjusted to up 10.5 and put 3 shots 6" high. Adjusted to up 9.5 and was almost dead on as far as elevation. Windage was off but it was blowing +/- 10. Using a Sightron III with 1/4 adjustments. The groups at each adjustment were pretty consistent--roughly 3-4 inch groups. So why am I so far off from the elevation adjustments indicated by the ballistic software? Thanks for any help!

[Altair]

You didn't mention the range your were shooting at, just the 100 yard zero, but according the ballistic calculator I use the come up is 11.69MOA at 500 yards, so I assume that is the range you were shooting at.

There are a couple reasons your data could be off. Did you chronograph the loads? They may not be exactly 2700fps. If they are going 2900fps the come up is 9.51MOA. Another thing that could affect it is that BC is not static with velocity and atmospheric conditions. If you look up BC on Sierra's website they list different BC's for different velocities to compensate somewhat for this. I think they are the only company that does that. Published BC is really just an estimate for bullet comparison and to get you in the ball park. There is no substitute for actually shooting your rifle and learning your combo at the distance you will be shooting.

[Primer]

You are correct, I was shooting at 500yds and feel like an idiot for not including that piece of the puzzle. Unfortunately, i do not have a chronograph--but i do have a birthday coming up! Thanks very much for taking the time to evaluate my question. Seems the best answer is shoot more and get to know the equipment better. Thanks again.

[RKG]

I can think of four factors that may explain an observed different between the ballistic flight projected by a typical ballistics software application and actual shots fired. In descending order of influence:

Error in muzzle velocity. As others have noted, you are estimating (or guessing at) your load’s muzzle velocity out of your rifle. A difference between actual muzzle velocity and the muzzle velocity input into the ballistics program will have a significant impact on bullet path. Taking a value for muzzle velocity from a load book table is only estimating; for identical bullets and propellant charges, differences in chamber dimension, barrel length, rifling characteristics and (perhaps most important) bullet “jump” distance to the lands will produce differences in actual muzzle velocity. The only accurate way to learn the velocity of your loads is to chronograph them and then adjust for the difference between velocity at instrument distance and muzzle velocity.

Error in ballistic coefficient. A “ballistic coefficient” is a modifier applied to the standard G1 drag coefficient that accounts for the effect of drag on a particular bullet’s shape, and which is employed in virtually all of the regularly available ballistic applications. However, since parasitic drag is a function of missile velocity (actually, the square of missile velocity), and given that the velocity of the bullet begins to decay the moment it exits the muzzle and continues to decay for the entire duration of flight, it stands to reason that a single value for ballistic coefficient will be both an estimate and a compromise. The only way to develop accurate ballistic coefficients for a bullet at a given muzzle velocity is to measure actual time of flight (or end-of-segment velocity) at several segments of the bullet’s flight, which is a difficult thing for most folks to do. As previously noted, Sierra reports multiple BC’s for its bullets at different instantaneous velocities, which implies some greater fidelity, but I have no idea how these are derived.

Error in ambient conditions. Ambient temperature, barometric pressure and humidity will have an effect on the density of the air through which the bullet flies, and, thus, on the effective drag coefficient. (In the aircraft world, the sum of these three factors, which have an effect on engine performance, is known as “density altitude.”) In addition, ambient temperature may also have an effect on muzzle velocity, given identical loads fired from the same rifle.

Error in sight line attitude. Most ballistic programs assume that the line of sight for the specific zero is horizontal. If the actual line of sight points uphill or downhill, the effect of gravity on both velocity decay and bullet drop from bore line will vary. For most people shooting from a bench at a known distance rifle range, this factor is quite small.

Bottom line: a ballistic program is intended to get you more or less on paper when holding over for a range other than zero. The way to find out with any precision how much holdover you really need for your load fired out of your rifle under the conditions of interest is fire some of your loads out of your rifle under those conditions and then record the result.

[Senderofan]

I think RKG covered the important points very well. I would second the idea of purchasing a chronograph...that will really help your cause. I've been using a program for several years....it is very accurate with my data...if I do my part. If I've made an error in altitude, temp and the like...I will see a deviation from my data from about 375 yards and beyond.

Hang in there....get the chronograph and you'll be rewarded with some accurate data!

Wayne

[GaCop]

The chronograph data will also tell you how well your loads are performing and whether they will shoot well at long range (low Sectional Density-SD and Extreme Spread-ES).

[Altair]

The chronograph data will also tell you how well your loads are performing and whether they will shoot well at long range (low Sectional Density Standard Deviation-SD and Extreme Spread-ES).


Sectional density has to do with bullet construction (ratio of mass to cross sectional area).

Standard deviation is a statistical term used to measure consistency (or lack of it). Lower is better.

[diriel]

The thing about JBM that many folks do not think about, is that it has several settings that can drastically affect POI. Height above bore for scope, Elevation above sea level, Temperature, ect ... these things DEFINITELY affect your POI. Make sure you input ALL the pertinent data right. Then you will find that JBM comes pretty close.

What you put into JBM affects what you get out of it...

Happy shooting,
Gary

[GaCop]

Thanks for correcting that. I meant "Standard Deviation" LOL!