Observed ballistic coefficient of 123 AMAX

[calshipbuilder]

Ive been pushing my grendel past 800 yards for a while. I use the shooter app on iphone and Android. I have always had problems getting past 600 yards. Today i decided to change the one variable I hadn't yet. Locking in environmentals from my kestrel 4500 and ranging out to targets at 875 and 975, I started to lower the BC till it matched what I saw on the targets. I'm going to use a G1 BC of .475 for now. Tomorrow I will test it at 1050 yds and see if it still works, then make sure its still valid at middle distances. I really like the performance of the 123 amax, but need reliable ballistics data for it. My plan is to develop a banded G1 for it, and maybe a G7 if I can ever firgure out how to do that. So, I'd like the hoard's help to nail down the BC.

Please reply to this thread with the following info:

Distance target was shot.
Muzzle velocity
Come up (elevation) increase from zero
Zero distance
Elevation
Pressure
Temp
Humidity
Wind speed and direction
Barrel twist

Thanks!

[bwaites]

Ive been pushing my grendel past 800 yards for a while. I use the shooter app on iphone and Android. I have always had problems getting past 600 yards. Today i decided to change the one variable I hadn't yet. Locking in environmentals from my kestrel 4500 and ranging out to targets at 875 and 975, I started to lower the BC till it matched what I saw on the targets. I'm going to use a G1 BC of .475 for now. Tomorrow I will test it at 1050 yds and see if it still works, then make sure its still valid at middle distances. I really like the performance of the 123 amax, but need reliable ballistics data for it. My plan is to develop a banded G1 for it, and maybe a G7 if I can ever firgure out how to do that. So, I'd like the hoard's help to nail down the BC.

Please reply to this thread with the following info:

Distance target was shot.
Muzzle velocity
Come up (elevation) increase from zero
Zero distance
Elevation
Pressure
Temp
Humidity
Wind speed and direction
Barrel twist

Thanks!

G1 BC's really don't reflect well on the BT bullets. Try using a G7 BC of .247 with your calculations and see how close it matches your observed data. That's the BC I use, and my actuals are pretty close to my calculations.


Distance target was shot. 375-700 yards
Muzzle velocity 2670 FPS
Come up (elevation) increase from zero Zero at 375 500 up 4.32, 550 up 6.24, 600 up 8.29, 650 up 10.46, 700 up 12.77. 750 up 15.23, 800 up 17.85, 850 up 20.64, 900 up 23.62, 950 up 26.81, 1000 up 30.22.
Zero distance 375
Elevation 2900
Pressure
Temp 60-75 degrees F
Humidity 50-60%
Wind speed and direction 5-15 variable
Barrel twist 8.75

Also, the lower muzzle velocity of the Grendel plays some part, since G1 BC's are based on flat based bullets driven at typically higher speeds. BC is variable to some extent based on velocity.

The 375 yard zero is because that was the first steel at Boomershoot, and I was rezeroing rifles after winter teardowns. Out to 700 the results are observed, beyond that they are calculated.

PS....Bryan Litz says the G7 BC on the 123 SMK is .260, and the listed G1 on it is .510, but variable based on speed. I don't think the G1 on the Hornady is as good as the Sierra. Unfortunately, Litz hasn't given a G7 on the 123 Amax yet. He lists the 120 Berger as a G7 of .232 and that places the Amax in between, which fits what I see when I look at them, for whatever the eyeball is worth!

[Raider1v1]

G1 BC's really don't reflect well on the BT bullets. Try using a G7 BC of .247 with your calculations and see how close it matches your observed data. That's the BC I use, and my actuals are pretty close to my calculations.


Distance target was shot. 375-700 yards
Muzzle velocity 2670 FPS
Come up (elevation) increase from zero Zero at 375 500 up 4.32, 550 up 6.24, 600 up 8.29, 650 up 10.46, 700 up 12.77. 750 up 15.23, 800 up 17.85, 850 up 20.64, 900 up 23.62, 950 up 26.81, 1000 up 30.22.
Zero distance 375
Elevation 2900
Pressure
Temp 60-75 degrees F
Humidity 50-60%
Wind speed and direction 5-15 variable
Barrel twist 8.75

Also, the lower muzzle velocity of the Grendel plays some part, since G1 BC's are based on flat based bullets driven at typically higher speeds. BC is variable to some extent based on velocity.

The 375 yard zero is because that was the first steel at Boomershoot, and I was rezeroing rifles after winter teardowns. Out to 700 the results are observed, beyond that they are calculated.

PS....Bryan Litz says the G7 BC on the 123 SMK is .260, and the listed G1 on it is .510, but variable based on speed. I don't think the G1 on the Hornady is as good as the Sierra. Unfortunately, Litz hasn't given a G7 on the 123 Amax yet. He lists the 120 Berger as a G7 of .232 and that places the Amax in between, which fits what I see when I look at them, for whatever the eyeball is worth!

thats interesting about the g1 vs g7 bc's.

what recipe are you using to get over 2600 with these? i am at 28.9 of varget w/ a cci large rifle primer and getting 2480 from a 24" barrel.

[bwaites]

thats interesting about the g1 vs g7 bc's.

what recipe are you using to get over 2600 with these? i am at 28.9 of varget w/ a cci large rifle primer and getting 2480 from a 24" barrel.

I have one of the few 28" barrels, and thats the factory Hornady AMAX load.

You won't get the velocities out of Varget that you want in the Grendel. You'll need to use Ramshot TAC, AA2520, XBR8208, BLC-2, or CFE223 to get decent velocity. Hornady uses a proprietary Superperformance type powder in their Grendel loads. I can come really close with CFE223 and XBR, but be very careful, especially with XBR. I've made quite a few belted Grendel cases chasing max velocity from that barrel using 123's! It shoots 107 Sierras and 108 Scenars really well, so I've pretty much decided to just use them.

[LRRPF52]

I've been using Strelok on my phone, and I recorded the pressure and temperature data when I zeroed. I make it a point to enter my pressure and temp data whatever the environment is, and so far, my POI has been dead-on with the 123gr A-MAX and 123gr SST, but I haven't shot past 600yds with those loads yet, as I just have a little 16" gun.

The German guy who posts those awesome long-range shooting/hunting videos uses a G7 BC for the 123gr Scenar that is somewhere in the mid-.2's as well, but lower than the published BC of .263 for G7.

[mongoosesnipe]

another consideration is bullet velocity most manufacturers only list one bc but sierra list multiple bc based on velocity
.264 dia. (6.5mm) 142 gr. HPBT MatchKing
.595 @ 2850 fps and above
.580 between 2850 and 2400 fps
.575 between 2400 and 2050 fps
.550 @ 2050 and below

based on my knowledge i would guess this i because of the shape of the pressure bubble made by the bullet during flight causing varying degrees of drag on the bullet dependent on its velocity

[tackdriver]

I also use Strelok and don't have a chronograph so curious as to what velocity you are getting out of your 16" w/ a-max factory loads

[Drifter]

I've found JBM Ballistics and Shooter to be spot-on for me using the 123gr A-max to 600 yards.

600 yards- Distance target was shot.
2500 fps- Muzzle velocity
4.4 mils- Come up (elevation) increase from zero
200 yards- Zero distance
100'- Elevation
29.84- Pressure
60 F- Temp
60%- Humidity
N/A- Wind speed and direction (Usually shoot on range protected from wind, but often need 0.1-mil correction for spin drift.)
1:8.75"- Barrel twist

2.7"- Sight Height (above bore)

[calshipbuilder]

Thanks for the good info thus far. From 0-600 the .510 g1 has worked just fine for me, but shooting past that, even to 650, has been problematic using .510. Thanks Bwaites for the .247 g7, I was still a few moa low at 1065, but close enough that I could see the dirt kick up. I have some more thinking to do, and am running out of factory ammo... My personal reloading failures mean I cant keep es tight enough for 1000 yard work.

[bwaites]

If you were just low, but your wind drift was close, chances are your velocity is actually optimistic. Most chronos I've seen tend to read higher than actual velocities.

[calshipbuilder]

Very possible, wanted to check my gun on a different chron, but my buddy forgot batteries :-/

[Drifter]

Comprehensive list of Sources of Ballistic Program Inaccuracies on this link:

http://www.arcanamavens.com/LBSFiles/Shooting/Downloads/Programs/

The closing statement on the link is noteworthy:

"When someone says that their un-tuned ballistic program was "right on at 1000 yards", I generally conclude that if they are telling the truth, they were lucky enough to have offsetting errors."

[bwaites]

Comprehensive list of Sources of Ballistic Program Inaccuracies on this link:

http://www.arcanamavens.com/LBSFiles/Shooting/Downloads/Programs/

The closing statement on the link is noteworthy:

"When someone says that their un-tuned ballistic program was "right on at 1000 yards", I generally conclude that if they are telling the truth, they were lucky enough to have offsetting errors."

Or as LR1955 told me at the very first class I ever attended with him, "Ballistics programs are great theoretical calculations, the problem with theory is that it doesn't run your bullet out there to the target!"

[BjornF16]

Comprehensive list of Sources of Ballistic Program Inaccuracies on this link:

http://www.arcanamavens.com/LBSFiles/Shooting/Downloads/Programs/

The closing statement on the link is noteworthy:

"When someone says that their un-tuned ballistic program was "right on at 1000 yards", I generally conclude that if they are telling the truth, they were lucky enough to have offsetting errors."

[bwaites]

I use Ballistic AE and cross reference with Bulletflight and iSnipe. I like Litz's numbers, but he doesn't have 123 Amax listed yet, so I've had to develop my G7 number. I'm REALLY anxious to see what he gets for a G7 BC on the Amax!

The truth is that out to 600 yards or so, I can't tell much real world difference between them using the official numbers and using my numbers, its only beyond that where I start to see differences.

As for using .1 Mil for spindrift, well, I can't tell 1-1.5 inches at 600 yards is due to spindrift or my hold! I don't use any spindrift adjustment out to 1200 yards with my 7mm WSM, because I can't calculate that into my numbers, its simply to small a value in comparison to wind, barrel variation, my hold, etc.