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308 168gr Amax changes in BC at long range

 
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  #1  
Old 10-26-2010, 03:47 PM
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308 168gr Amax changes in BC at long range

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.

Last edited by Kiwi Nate; 10-26-2010 at 05:21 PM. Reason: typo
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Old 10-26-2010, 05:59 PM
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Re: 308 168gr Amax changes in BC at long range

How did you determine this?
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Old 10-26-2010, 06:43 PM
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Re: 308 168gr Amax changes in BC at long range

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.
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Old 10-26-2010, 07:14 PM
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Re: 308 168gr Amax changes in BC at long range

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.
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Old 10-26-2010, 08:58 PM
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Re: 308 168gr Amax changes in BC at long range

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.
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Old 10-27-2010, 05:13 AM
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Re: 308 168gr Amax changes in BC at long range

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!
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Old 10-27-2010, 10:45 AM
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Re: 308 168gr Amax changes in BC at long range

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
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