308 168gr Amax changes in BC at long range

[Kiwi Nate]

Hi all, I have been working with the 168gr Amax in a .308. Twist rate is 1-12, bullet jump is the usual .100" so there will be a small amount of yaw or precessional rotation effecting BC at long range.

What I am noticing is that at 2100fps, the BC drops only slightly, from .475 to .450. Below 1600fps (600 yards), the BC of the Amax as fired in this rifle, decreases drastically, to .38.

Would like to hear from anyone who has experienced similar results.

 

[BryanLitz]

How did you determine this?

 

[Kiwi Nate]

Hi Bryan, I used my standard testing procedures.

First, work up an accurate load. In this instance, it is the 168gr Amax at 2777fps. ES is very low, grouping around 1/4MOA and less.

Step two, calibrate the scope. This scope is a Leupold and from scope to scope, I find that each MOA of adjustment can be anything from 1" to 1.3" with this brand so I have to be very careful about calibration. This scope was tested on a grid at 100 yards- the rifle mounted in a vice I have set up for this. The rifle was then test fired for confirmation. With this scope, one MOA elevation is actually 1.0235 (halfway between 1" and 1MOA).

Next is ensuring the correct inputs are entered into Sierra Infinity 6. The default environmental paremeters were fine. The scope height was set at 1.6" true to the scope. From I6, I take the data and load it into microsoft excel. With excel, I can automatically re-calibrate the MOA drop column to suit the error within the scope.

With all of this done, from this particular rifle that I am testing for a client, the drop charts are accurate to 500 yards or so but if I use the I6 velocity boundaries of .475, .475, .475, .475, the trajectory is off the mark. Drop at 750 yards is 1 minute lower than the standard I6 output.

 

[Kiwi Nate]

Should mention, if I change my scope calibration inputs to double check my procedures, the trajectory does not match any potential inputs in the field. The only way to match the actual trajectory is to change the BC.

 

[Kiwi Nate]

Another factor I haven't really looked into is- perhaps an accumulative error within the scope turret rather than a static error. Have made up some rounds so I better head out again but this time will take the laptop.

 

[Kiwi Nate]

Well, what a waste of a thread sorry guys. Was a very slight accumulative error in the turret. Took quite a bit of time shooting at 100 yard increments out to 750 yards. Have re-calibrated the scope and can now finish the job.

I tell you, some of these jobs really test the grey matter. I came inside the house after testing and went straight for the Nurofen to settle a headache that was coming on!

 

[BryanLitz]

Good on you for seeing this thru.

Field testing to determine BC, no matter what method is used, is an exercise in attention to detail.

For what it's worth, I got the following results for the 168 AMax:

velocity/G1 BC/G7 BC

3000/.483/.237
2500/.464/.233
2000/.449/.226
1500/.403/.224

The BC definitely drops off quite a bit for this bullet as it slows, but according to my results, it's not as much as you were seeing.

Good shooting,
-Bryan

 

[Kiwi Nate]

Thanks Bryan, always learning. Thanks for posting your BC's too.

 

[Michael Eichele]

Here is my observation on the 168 AMAX.

The ballistic calculators I use utilize a coefficient decay rate in place of drag models. The figure used to match G1 is .290. The higher the decay factor, the faster the BC is dropping. The lower the decay factor, the slower the BC is dropping. When I fired the 168 AMAX's over double chronies, I came up with .490 on the nose for G1. After setting up at 650 yards the first bullet and group drilled the center X. However, when firing out to 800 and 900 yards I have to use .500 for a decay rate with a BC of .490 to get the drops to match from 100-900 yards.

Its un-orthodox for sure but it works. In any event, it shows that the 168 AMAX does not hold it's BC as well as some other bullets.

M

 

[Scot E]

Well, what a waste of a thread sorry guys. Was a very slight accumulative error in the turret. Took quite a bit of time shooting at 100 yard increments out to 750 yards. Have re-calibrated the scope and can now finish the job.

I tell you, some of these jobs really test the grey matter. I came inside the house after testing and went straight for the Nurofen to settle a headache that was coming on!

This is one of the things that is really frustrating for guys that dial elevation. Even if due diligence is taken to determine correct click values those values often begin to change as one dials toward the maximum range of the scope, ie at longer distances. I have got to the point where I check click values up to the max the turret will move before my reticle stops moving. Having multiple inputs on a BC program would be nice for this. Otherwise you have to average the error over the entire shooting distance or have notes to compensate so many clicks for longer distances.