# estimate on BC to use with 180 eld-m with 1-9.25 twist

##### Well-Known Member
box says G7.406 1-8 twist

website says
0.391 G7
0.376 G7
0.368 G7 for 8.75 twist

0.411 G7
0.408 G7
0.394 G7 for 7.5 twist

i'm hucking them @ 2936fps... dont see why id have stability issues

but im new to "making up a BC"

help?

#### cjl2010

##### Well-Known Member
Try bergers twist rate calculator?

#### gohring3006

##### Well-Known Member
LRH Team Member
You need length of the bullet and make sure your elevation is accurate when using Berger’s calculator.

##### Well-Known Member
Like i said, im looking for a BC for 9.25 twist, not wether itl b stable... i live in british columbia 3000-7000ft elevation is typical shooting/hunting

... i ran my #’s through the program, everything seems fine

Just too many BC’s to pick from and NONE are for a 1-9 twist

#### cjl2010

##### Well-Known Member
If it’s unstable their calculator will give you a corrected BC.

#### gohring3006

##### Well-Known Member
LRH Team Member
You have to calculate stability to get a estimated BC.

#### Hand Skills

##### Well-Known Member
BC is not a static thing, it changes with respect to velocity and air pressure.

The best way to make up a BC is to shoot some data, and then tweak the BC in your firing solution until it matches what you see at the target.

If you start with a G3 of .37, that should get your predictions onto paper during testing.

When setting up a new firing solution, BC is the LAST thing I worry about.

Here is a quote from the "Long Rangers" article by Darin Cooper recently published on this site. He is speaking to rangefinders with built in ballistics software specifically, but the procedure is good for any firing solution
-----------------------

Calibration Tips:
1) Always use a 100-yard zero… ALWAYS! When you zero at 100 yards, it won’t matter if you’re at sea level and zero degrees, or 10,000 feet and 80 degrees; your zero should hold. If it gets bumped, you can adjust the scope back to zero without influence from atmosphere. On the contrary, if you zero at 300, your zero will change with air density and re-zeroing under different conditions will cause errors. You can still hunt with your turret dialed to 250 yards for quick offhand shooting. Use a G7 BC with modern, high-BC bullets or use a G1 BC with shorter-nosed, flat-based bullets and bullets with no boat tail. If using the KILO 2400, the custom curve will usually be the best option for BC.

2) After you have a solid zero, calibrate velocity by shooting targets at 300 and 600 yards. Modify only the velocity input until the rangefinder gives hold-over in your preferred unit (MOA, MIL, inches, or a shoot-to range) that centers your impact group vertically at 300 yards. Next, fine tune your muzzle velocity at 600 yards until impacts are again centered vertically. BC has little effect on trajectory at these shorter ranges, so as long as it is reasonably close and your rifle is consistent, you should have no problem getting on target between 100 and 600 yards.

3) Next, calibrate BC to correct your elevation at long range. I typically do this between 1000-1200 yards. You may need to validate at shorter range with lower velocity cartridges and light-for-caliber bullets to avoid trans-sonic and subsonic velocity where BC won’t predict drag accurately. As long as your bullet velocity is still above 1400 fps at the target, you should see consistent results. If you’re shooting a .308 Win. and 150-grain bullets, you may need to calibrate BC below 1000 yards. Conversely, if you’re shooting a .338 Lapua with 300-grain bullets, you may get a better calibration by truing BC at 1000 and 1500 yards.

If your impacts are 2 MOA low at 1000 yards, decrease the BC in increments of .005 in the rangefinder or app until it predicts a solution that is 2 MOA higher. Once you’re on target at this distance, your BC is verified. BC’s commonly require calibration adjustments of 3-10% up or down from advertised values. BC varies from published values due to measurement errors, muzzle velocity, barrel twist rate, and even bore diameter. Changing the BC should have very little effect on your impacts at 600 yards and closer. Truing the BC at long range allows for more accurate wind corrections even if you never intend to shoot beyond 500 yards, so it’s still worth the effort to validate it.

https://www.longrangehunting.com/articles/the-long-rangers.1203/

#### Mikecr

##### Well-Known Member
If it’s unstable their calculator will give you a corrected BC.
Which will be very near ZERO (whatever tumbling would amount to).

#### cjl2010

##### Well-Known Member
Which will be very near ZERO (whatever tumbling would amount to).
I’m not sure I follow, care to elaborate?

#### Mikecr

##### Well-Known Member
UNSTABLE = immediate tumbling, BC not applicable
Marginal stability can lower BC
Fully stable should allow full BC
Excess spin can affect bullet integrity and lower BC (if really bad)

##### Well-Known Member
so fps and altitiude makes up for a slower twist...

im playing with the hornady 4DoF calculater now...

#### Mikecr

##### Well-Known Member
so fps and altitiude makes up for a slower twist...
Less drag does, speed has so slight a positive affect(if any) that it won't ever bail you out of a poor stability condition.

Twist requirements are stated in displacement per turn. That displacement is a distance of air at some standard atmosphere. On muzzle release, a bullet will displace the same distance per turn, regardless of how quickly it covers that distance(muzzle velocity).
Now as a bullet slows downrange(from drag), it has not slowed in revolution rate by so much. This becomes less distance per turn, and effective gyroscopic stability goes up further & further down range. This, assuming dynamic stability, which takes a hit with poor muzzle release and again near Mach1(speed of sound).

Given all this, if you were to average stability at various distances to a target, the slower bullet could average higher, or lower, in stability. But there is by far less affect in this variance than that of baseline displacement per turn.
MV itself will never really help.

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