Your .264 140gr eldm bc?

[Macintosh]

What are folks using successfully for a g7 bc for 140gr eldm’s?
Hornady website says .326
The AB bullet library says .295, which is what Im using.

Have a situation where calibrating my calculator on a new barrel at +\- 600 is resulting in a trued velocity about 60 fps faster than chrono’d velocity. Chronod velocity is 2598, trued velocity is 2660. If I used a higher bc it would make up for some of this difference. The trued velocity lines up with reality so its what Im using for now, Im just mostly curious what other folks have successfully used and what others have done in this situation.

 

[wpeach1912]

When you start truing velocity and BC weird stuff happens. Run your measured velocity and just true BC. Obviously make very certain that all the other inputs (atmospherics, rifle setup, etc.) are input right. I have had the ELDMs true up between .285-.300. Also, the more rounds you can shoot to get a solid idea of your cone of fire the better

 

[Macintosh]

Zero as well as center of groups at distance were all done using 10-round groups. Average 10-round 100yd group is just over 1moa, as are groups at distance.

Adjusting the BC to hornady’s published .326 and using the chrono velocity does put my actual hits much closer to the calculated solution, they are about .1 mil apart at that point. Usually bc isnt enough, which I believe is why Ive always been taught to not touch bc until at a distance near transsonic. In this case thats really my question, given the wide variation in BC #’s from different sources. I had assumed the AB doppler numbers were more accurate, but…

 

[Jordan Smith]

Zero as well as center of groups at distance were all done using 10-round groups. Average 10-round 100yd group is just over 1moa, as are groups at distance.

Adjusting the BC to hornady’s published .326 and using the chrono velocity does put my actual hits much closer to the calculated solution, they are about .1 mil apart at that point. Usually bc isnt enough, which I believe is why Ive always been taught to not touch bc until at a distance near transsonic. In this case thats really my question, given the wide variation in BC #’s from different sources. I had assumed the AB doppler numbers were more accurate, but…

You should be adjusting the quantity you have the least confidence in. If you're measuring muzzle speed with a single shot and a Chrony F1, or not measuring it at all, then yeah, the muzzle speed value is suspect and should be trued if the solution diverges from DOPE. But if you're calculating the average muzzle speed based on 20+ shots measured with a Xero or LR, I'd have fairly high confidence in that value. Remember that BC varies slightly with each barrel and even bullet lots, so what someone else measured is just a starting point and is usually the variable with the most uncertainty (given current Doppler chronos, Kestrels, etc., to precisely measure other variables).

 

[Macintosh]

You should be adjusting the quantity you have the least confidence in. If you're measuring muzzle speed with a single shot and a Chrony F1, or not measuring it at all, then yeah, the muzzle speed value is suspect and should be trued if the solution diverges from DOPE. But if you're calculating the average muzzle speed based on 20+ shots measured with a Xero or LR, I'd have fairly high confidence in that value. Remember that BC varies slightly with each barrel and even bullet lots, so what someone else measured is just a starting point and is usually the variable with the most uncertainty (given current Doppler chronos, Kestrels, etc., to precisely measure other variables) .

That makes perfect sense, thank you. In this case the velocity is only a 5-round average but measured with a garmin and had a low sd, so it should be at least decent. Sounds like using the chrono velocity and adjusting bc is better in this case.
My question then is hornady’s published .326 gets me very close, but not exact. I’d have to use a bc of .360 to get the calculator to line up perfectly with actual hits, and that just doesnt seem like it can be right. I’m pretty confidant in the rest of the inputs. So which would you adjust from here as I further refine this, velocity or bc? Among other things this barrel only has about 80 rounds on it, so Im guessing I’ll need to adjust a bit in the not too distant future. Without a chrono on hand would you true velocity next knowing it’s likely to change as the barrel breaks in?

 

[SouthPaw]

I use Hornady's mach 1.75 BC value of .310 G7 out to 1100 yards for the 140gr ELDM from multiple 6.5CMs, and it waterlines perfect with velocity values off the garmin.

Shooting 600 yards with a .295 G7 will raise your trued velocity becuase AB is averaging BC down to <1500fps, way way past the 600 you are shooting.

 

[Jordan Smith]

That makes perfect sense, thank you. In this case the velocity is only a 5-round average but measured with a garmin and had a low sd, so it should be at least decent. Sounds like using the chrono velocity and adjusting bc is better in this case.
My question then is hornady’s published .326 gets me very close, but not exact. I’d have to use a bc of .360 to get the calculator to line up perfectly with actual hits, and that just doesnt seem like it can be right. I’m pretty confidant in the rest of the inputs. So which would you adjust from here as I further refine this, velocity or bc? Among other things this barrel only has about 80 rounds on it, so Im guessing I’ll need to adjust a bit in the not too distant future. Without a chrono on hand would you true velocity next knowing it’s likely to change as the barrel breaks in?

I'd continue gathering velocity data and use the measured values as the barrel continues to speed up. The more you measure muzzle speed, the more confident you can be in that value. Essentially, trust the values that are measured with a high confidence, and true the variables that you have the least confidence in with your specific rifle/bullet setup. If you are truing either muzzle speed or BC, I would also make sure to do it at distances approaching as close to the bullet's transonic zone as possible. It's quite possible that the effective BC out to 600 yards is 0.360 instead of the published 0.326, but that value may decrease as you approach the transonic range, requiring some averaging.

 

[khuber84]

I use Hornady's mach 1.75 BC value of .310 G7 out to 1100 yards for the 140gr ELDM from multiple 6.5CMs, and it waterlines perfect with velocity values off the garmin.

Shooting 600 yards with a .295 G7 will raise your trued velocity becuase AB is averaging BC down to <1500fps, way way past the 600 you are shooting.

I've ran 0.310 for the 140 eldm as well, this was for a waterline at 1230y. Oddly enough the 147 was only 12 points higher for waterline at the same distance.

 

[Formidilosus]

What are folks using successfully for a g7 bc for 140gr eldm’s?
Hornady website says .326
The AB bullet library says .295, which is what Im using.

Have a situation where calibrating my calculator on a new barrel at +\- 600 is resulting in a trued velocity about 60 fps faster than chrono’d velocity. Chronod velocity is 2598, trued velocity is 2660. If I used a higher bc it would make up for some of this difference. The trued velocity lines up with reality so its what Im using for now, Im just mostly curious what other folks have successfully used and what others have done in this situation.

It is AB- the values they give most Hornady bullets will result in high hits. I have written multiple times about this. AB is wrong on most Hornady bullets. 4dof produces silly outputs for non Hornady bullets.


BC variation differences between barrels/guns is small. It is not 50+ G1 BC points- it is not enough to see in the ye to al range of nearly all cartridge/bullet combinations.

 

[entropicitis]

Are you using a Garmin to get velocity? Once I started doing that I never need to true MV. If you have a elite Kestrel you might consider using the custom curve rather than BC.

 

[Macintosh]

Are you using a Garmin to get velocity? Once I started doing that I never need to true MV. If you have a elite Kestrel you might consider using the custom curve rather than BC.

no chrono, no kestrel. In this case, I got a new barrel cut and threaded at 20" for my spankin new suppressor, and a couple flats of hornady match ammo, and plugged in the AB regular BC. While zeroing at the range, a guy I was chatting with had a garmin chrono, so I asked if I could get a quick velocity, so I zipped off 5 rounds to get an average and was done. Usually I plug in box velocity minus 25fps/inch below 24", the custom drag curve from AB for a BC value, and true it at the longest range I can manage (usually around 600 yards these days, long ranges are not easy to come by in the east). Then true the velocity to my actual impacts at range, and the result has been quite good for my casual use, and plenty good enough at the ranges I hunt. But in this case having an actual velocity combined with having two widely divergent BC figures, left me questioning the best way to go about doing this.

 

[S.Clancy]

140 ELDM's out of my Kimber are pretty much dead-on BC. I don't know how BC could be that far off, my guess is error in your chrono

 

[bojangles808]

It is AB- the values they give most Hornady bullets will result in high hits. I have written multiple times about this. AB is wrong on most Hornady bullets. 4dof produces silly outputs for non Hornady bullets.


BC variation differences between barrels/guns is small. It is not 50+ G1 BC points- it is not enough to see in the ye to al range of nearly all cartridge/bullet combinations.

What bc do you use for 147 and 108 eldms when inputting to ab?