Truing BC

[Kramerica]

Sierra 130 game change G1 BC is .510 at 2350FPS according to Sierras website. If I’m getting 2775FPS what is the G1 BC now? Is there a rule of thumb or formula to get that corrected BC?

 

[clowdis]

The G1 BC is still .510. If you're trying to figure drop and windage you can do that here http://www.jbmballistics.com/cgi-bin/jbmtraj_simp-5.1.cgi

 

[Kramerica]

Maybe I’m wrong here but from what I understood the BC is only valid at a specific velocity. As you get away from that velocity your BC changes. Also the picture I’ll upload shows different BC’s at different velocities for the same bullet as it slows...

 

[clowdis]

You can read more here: https://simple.wikipedia.org/wiki/Ballistic_coefficient

 

[AllenOne1]

Jalen
You are correct your BC will change based on velocity, altitude, temp, etc. I'm not sure how to calculate the true BC without shooting to get your actual drop and then playing with the BC values to match your field data.

 

[Kramerica]

Jalen
You are correct your BC will change based on velocity, altitude, temp, etc. I'm not sure how to calculate the true BC without shooting to get your actual drop and then playing with the BC values to match your field data.

I was more interested in True BC for figuring out what mph gun to get quick wind holds. Elevation part is easy enough. Thanks for the input

 

[linebaugh]

To answer your question it's .510 @ 2350 and above.... so .510

It will likely be off up or down by a small amount. Make sure to enter all your corresponding data to gather the actual BC as precisely as possible. Your dope will only be as good as the effort put into gathering it.

And your elevation and wind calls both rely on the same number.

 

[Ranger188]

If you use the JBM calculator it compensates for that.
Plug in the Sierra bullet, hit calculate and then look at the top of the chart,
it shows the different B.C. that it's using for the drop in speed.

 

[Ranger188]

Trajectory (Simplified)Input DataManufacturer:SierraDescription:HPBT MatchKing™Caliber:0.243 inWeight:107.0 grBallistic Coefficients:0.495 [0-1600], 0.509 [1600-1800], 0.522 [1800-2500], 0.527 [2500-5000] G1 (ASM)
Unless Litz has tested it and then he has a G7 BC

 

[284winner]

BC drops as velocity drops. Sierra's 2350 fps number is pretty slow. For a game bullet of 130 grains and a decent BC, unless your shooting a 6.5 Grendel, you'll be much higher in muzzle velocity. Starting out at 3000 fps, at 400 yards your BC will be about the same.

 

[mikecr]

Sierra 130 game change G1 BC is .510 at 2350FPS according to Sierras website. If I’m getting 2775FPS what is the G1 BC now? Is there a rule of thumb or formula to get that corrected BC?

If declared G1 BC is accurate, then looking up the G1 standard curve to new speed drag coefficient leads to .523 for new G1BC. This, given the only change is muzzle velocity.

There is no rule of thumb(that I'm aware of), as here drag calcs for the bullet at speed are divided by G1 drag coefficient at speed to get a form factor which the sectional density is divided by..
Also, if this bullet is not well matching to the G1 drag model (many are not) then errors grow across a gamut of speeds, leading to the only REAL & KNOWN G1 BC being at specific points that it was verified at.
Many are migrating to G7 BC for a better match across velocity span.
Do you have the G7 BC?

 

[DaveTooley]

I was more interested in True BC for figuring out what mph gun to get quick wind holds. Elevation part is easy enough. Thanks for the input

If your elevation numbers are good then using the same BC number will be accurate for wind holds. Needing a very very accurate BC number isn't required until you get past 600 yds. even then you can't see it on the target till much further out. So much gets lost in the noise up close. I have dopplar data on quite a few bullets out to 2500 yds. There is a change in BC over that distance but it's very small. So small that as speed decreases atmospheric conditions will completely hide any change in BC on the target. When I get to the shop I'll post some numbers.

 

[CharlieNC]

Jalen the wind variability and your inability to read it from muzzle to target will over shadow a small BC difference.

 

[DaveTooley]

Here are some numbers for a comparable 6.5 bullet. I'll list the G1 BC from 100 to 1000 yds.
At 900 it's in or close to the transonic zone and at 1000 yds it's in the transonic velocity range and the BC drops quickly.
1-.579
2-.572
3-.571
4-.572
5-.568
6-.562
7-.553
8-.537
9-.509
10-.466
These numbers are accurate for that day with that specific rifle/barrel combination. Different day, different rifle, different atmospherics and the numbers will change.

 

[epc308]

I only know enough to be a danger to myself, so I called Sierra when I needed info for an unknown distance shoot at variable targets in a farm field. He asked for chrongraphed velocity, which bullet I was using, and the approximate distances. He dropped numbers in his confuser and read them off to me. Be sure to let them know If you have a MOA scope base and what angle it is because that makes a difference. His data let me know how high I needed to zero at 200 to be close. I called Burris with that info and they gave me values for the hash marks on my scope. I spent an hour and a half on the phone with the two calls, but it all worked as I connected with the targets that year.

 

[Kramerica]

If declared G1 BC is accurate, then looking up the G1 standard curve to new speed drag coefficient leads to .523 for new G1BC. This, given the only change is muzzle velocity.

There is no rule of thumb(that I'm aware of), as here drag calcs for the bullet at speed are divided by G1 drag coefficient at speed to get a form factor which the sectional density is divided by..
Also, if this bullet is not well matching to the G1 drag model (many are not) then errors grow across a gamut of speeds, leading to the only REAL & KNOWN G1 BC being at specific points that it was verified at.
Many are migrating to G7 BC for a better match across velocity span.
Do you have the G7 BC?

I get what your saying. I was just looking at those BCs at different velocities and thought since my Muzzle velocity was so much higher that maybe it’d change more. I know G7 is the standard for modern bullets. I was trying out the MPH gun method using G1BC that frank from snipers hide put out. So I wanted a solid G1BC to start

 

[Ned Ludd]

Maybe I’m wrong here but from what I understood the BC is only valid at a specific velocity. As you get away from that velocity your BC changes. Also the picture I’ll upload shows different BC’s at different velocities for the same bullet as it slows...View attachment 1185114

Not sure what you're not getting about the BC. Sierra provides 3 different velocity ranges that cover any velocity you might observe from zero on up, and three different G1 BCs, one for each velocity band. Your muzzle velocity falls into the highest velocity band, so use the 0.510 G1 value and you will be fine.

This is the primary reason that Bryan Litz at Berger Bullets has been advocating the use of G7 BCs for years. The G7 BC is derived in part by comparison to the G7 Standard bullet/shape, which is a much closer match the shape, and therefore the drag curves, of the bullets we actually use. For that reason, G7 BCs are relatively insensitive to velocity and don't typically need to be corrected for different velocity bands. The G1 Standard is shaped more like a flat base pistol bullet, and thus the shape and drag curves derived using it must be corrected within different velocity bands. See image below:





FWIW - for a wide range of lead core bullets, multiplying the G1 BC by 0.51 will provide a G7 BC value very close to the actual measured G7 BC. In a pinch, you can always go that route, but you have to remember to change the BC selection in your ballistic program to G7.

As a learning exercise, you might try making a drop chart using each BC (.510 G1; 0.260 G7), then determine how your actual drops match up with both.

 

[338 dude]

Trajectory (Simplified)Input DataManufacturer:SierraDescription:HPBT MatchKing™Caliber:0.243 inWeight:107.0 grBallistic Coefficients:0.495 [0-1600], 0.509 [1600-1800], 0.522 [1800-2500], 0.527 [2500-5000] G1 (ASM)
Unless Litz has tested it and then he has a G7 BC

That bullet is not listed in his third edition of ballistic performance of rifle bullets I don’t know if there’s a fourth yet or not

 

[Kramerica]

Not sure what you're not getting about the BC. Sierra provides 3 different velocity ranges that cover any velocity you might observe from zero on up, and three different G1 BCs, one for each velocity band. Your muzzle velocity falls into the highest velocity band, so use the 0.510 G1 value and you will be fine.

This is the primary reason that Bryan Litz at Berger Bullets has been advocating the use of G7 BCs for years. The G7 BC is derived in part by comparison to the G7 Standard bullet/shape, which is a much closer match the shape, and therefore the drag curves, of the bullets we actually use. For that reason, G7 BCs are relatively insensitive to velocity and don't typically need to be corrected for different velocity bands. The G1 Standard is shaped more like a flat base pistol bullet, and thus the shape and drag curves derived using it must be corrected within different velocity bands. See image below:


View attachment 1185341


FWIW - for a wide range of lead core bullets, multiplying the G1 BC by 0.51 will provide a G7 BC value very close to the actual measured G7 BC. In a pinch, you can always go that route, but you have to remember to change the BC selection in your ballistic program to G7.

As a learning exercise, you might try making a drop chart using each BC (.510 G1; 0.260 G7), then determine how your actual drops match up with both.

I was looking at it thinking there must be a higher band. I get it now I’m in the highest one. Like I said before I wanted a good G1 BC number to try a different wind holding method. Thanks for the info

 

[clowdis]

This is the primary reason that Bryan Litz at Berger Bullets has been advocating the use of G7 BCs for years. The G7 BC is derived in part by comparison to the G7 Standard bullet/shape, which is a much closer match the shape, and therefore the drag curves, of the bullets we actually use. For that reason, G7 BCs are relatively insensitive to velocity and don't typically need to be corrected for different velocity bands. The G1 Standard is shaped more like a flat base pistol bullet, and thus the shape and drag curves derived using it must be corrected within different velocity bands. See image below:


View attachment 1185341


FWIW - for a wide range of lead core bullets, multiplying the G1 BC by 0.51 will provide a G7 BC value very close to the actual measured G7 BC. In a pinch, you can always go that route, but you have to remember to change the BC selection in your ballistic program to G7.

As a learning exercise, you might try making a drop chart using each BC (.510 G1; 0.260 G7), then determine how your actual drops match up with both.

If my memory serves me correctly I think that the G1 BC formula's were developed for calculating the trajectories of artillery projectiles. Guess it works best for 15 mile shots! This is the reason that Brian prefers the G7 BC's, because they are more relevant to the projectiles we use.