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.338 270gr ELDx verified bc results

300 Driver said:
I'm looking for real world data on the G7 BC for the 270gr eldx

I recently verified drops at 600, 800, and 1000yds and had to drop the BC from factory claimed .381G7 to a .345G7 to make my ballistic app match real world. I could also make the app match by dropping 40fps from my 5-shot chrony avg and keeping the advertised bc.

Bullet Drop plus app
1000' MSL
86% humidity
78F
30.12

33SM @2748fps off a magneto speed.

I'd like to hear what everyone else uses for this bullet's G7. Thanks in advance!
Click to expand...
Hello there, I'm shooting that projectile from a 338LM at 2798 and the BC is .380G7 for me. I have trued out to 980yds and it is still good to go. I use AB for my software as well as JBM ballistics.

I know from talking to Hornady engineers that they have 3 windows they use for BC calculations but I don't remember the exact ranges. It was like 2500+, 1800-2500, 1800- FPS and then they did some whiz bang math and assigned an average BC.

Hope that helps!
 
300 Driver said:
First, thank you for the link. I was not aware of that data. It seems to confirm that my adjusted BC of .345 is significantly low.

As to what you and MM and RockyMt have stated, I do not disagree with the information you all have stated. But it did not answer my question. It did answer how variables affect BC calculations and why that happens, but my questions is what BC are others using. Have you actually used the .338cal 270gr ELDx? If so, What data did you input for BC into your ballistic app?

I believe I have an equipment problem I am trying to troubleshoot. Maybe my Magnetospeed is no longer within specs, maybe my ballistic app sucks, or maybe others see SIMILAR, NOT EXACT data to my experience and I'm just fine. I'll look into the AB app and the custom curve, that sounds pretty cool, always happy to find a more accurate solution.

As to trueing habits, this is my process after zeroing with a chosen load. Maybe this is contributing?
1) input advertised G7 BC and 5-shot chrony avg velocity plus environmental from kestrel and gun details into app
2) shoot at 600, 800, and 1000 and record DOPE ( repeat this step at least 3 times over different days) in field conditions
3) adjust BC or speed, but not both, to get app to match real world data.
I commonly harvest game in the 800 and less range, hence the chosen ranges.

I read more than I type on his forum and I think the educational information can be awesome, but I think the previous responses jumped to education, when all I asked for was data. I have reviewed my DOPE again and realized my wind inputs were very close(within my shooting capabilities), it was only the elevation that needed adjustment. I am leaning towards chrony error.
Click to expand...
It's because the answers we're giving you is actually case. If you want a bias confirmation, then no, I haven't used specifically the .338 in that grain in that specific bullet. So I cannot confirm that mine is also low.

Yes how your trueing is what's effecting it.
With that round you should only be adjusting your velocity. Find where transonic is, shoot a little before that. Say it 950y.

If you're kestrel ab, you would use DSF for subsonic. If not, try using that different mach publishings that I linked for BC. Match the mach your distance truing.


As for chrono errors, it rough trying to use a 5 shot average. Especially if the barrel is new.

The good news is, you can basicly guess the velocity by truing like I said above. If it takes 4.2 mils to hit a 8 inch plate at 950y use the mach adjustment for BC that Hornady published, then adjust your velocity till it matches 4.2 mils.

Added: you can try this as well. Back in the day when PRS was new, people had good luck doing this to get things to line up state to state. https://www.jbmballistics.com/cgi-bin/jbmbcv-5.1.cgi
 
Last edited:
RockyMtnMT said:
I can tell you that I have seen much bigger variations in bc from rifle to rifle with the same bullet than what you are seeing. I have been beat up on this sight for it many times. The fact is it happens to any bullet and bc is not a constant.

My guess is your mag speed is still good but I highly recommend the Garmin. It is so easy!
Click to expand...
This is great to hear, especially given the volume of your experience! I haven't seen this large of a change when BC trueing before.

Barbourcreek said:
Download my app and you can make your own BC for your gun
Eagle Ballistics
Click to expand...
I'll check it out! Great video, essentially how I true my BC as well. It was my final result that seemed odd. But your app makes the process simpler than the one I've been using.

Thanks!
 
Something else that came to mind. If you are not spot on with your zero it can throw your bc calculated impacts off quite a bit at distance. Bottom of the dot at 100y and all of a sudden you are 5" low at 800y.
 
You may wish to consider the following:

1) The purpose of applied mathematical analysis is a decision not a number.
2) I believe the real decision you are trying to make is - At some future point, I want hit a 8 to 14 inch target at some random distance (for example 842 yards) just how many 1/4 moa clicks do I need to dial to given my 100 yard zero.
3) For relatively high BC bullets, drop within each 100 yard increment is reasonably linear, less than a 1/4 MOA from a straight line.

Therefore, if I were trying solve this problem; I would calculate my drop chart using the .345 G7 because that is what the rifle is actually doing. You can also then validate at 700 and 900.

In summary, it really doesn't matter whether the BC is .345 or.381 or somewhere in between. What matters is creating and accurate drop chart and since drops are relatively linear and you have 3 data points and can validate two more you can create a drop/click chart within a 1/4 of an MOA.
 
dfanonymous said:
It's because the answers we're giving you is actually case. If you want a bias confirmation, then no, I haven't used specifically the .338 in that grain in that specific bullet. So I cannot confirm that mine is also low.

Yes how your trueing is what's effecting it.
With that round you should only be adjusting your velocity. Find where transonic is, shoot a little before that. Say it 950y.

If you're kestrel ab, you would use DSF for subsonic. If not, try using that different mach publishings that I linked for BC. Match the mach your distance truing.


As for chrono errors, it rough trying to use a 5 shot average. Especially if the barrel is new.

The good news is, you can basicly guess the velocity by truing like I said above. If it takes 4.2 mils to hit a 8 inch plate at 950y use the mach adjustment for BC that Hornady published, then adjust your velocity till it matches 4.2 mils.

Added: you can try this as well. Back in the day when PRS was new, people had good luck doing this to get things to line up state to state. https://www.jbmballistics.com/cgi-bin/jbmbcv-5.1.cgi
Click to expand...
Good copy. I did true for velocity as well. That yielded 2708fps, but I have always understood truing for BC to be more accurate.

After the earlier suggestion, compared to 4DOF online and it matches my actual drops much closer than my app. I only use my app to get started on confirming drops. I use confirmed drops for hunting, it's faster for me. But if I travel, I'd like my app to be more accurate for changing conditions. To date, I have only used bow when traveling to hunt.

Either way, I'm confident my DOPE is spot on for the shooter and the combo to my 800yd limit, so all is good in the world. Thanks to all for the insight and the data others have observed.
 
300 Driver said:
Good copy. I did true for velocity as well. That yielded 2708fps, but I have always understood truing for BC to be more accurate.

After the earlier suggestion, compared to 4DOF online and it matches my actual drops much closer than my app. I only use my app to get started on confirming drops. I use confirmed drops for hunting, it's faster for me. But if I travel, I'd like my app to be more accurate for changing conditions. To date, I have only used bow when traveling to hunt.

Either way, I'm confident my DOPE is spot on for the shooter and the combo to my 800yd limit, so all is good in the world. Thanks to all for the insight and the data others have observed.
Click to expand...
Accurate by who? It depends where you are having problems. BC has little to no effect on impacts within 600 for most cartridges.
BC starts to become noticeable with compounding error at around 800 or so. That's when you would tweak your BC. This is so you can have it lined up to extreme ranges.

Even if I'm just a stranger on the internet whose background you don't know, it's actually how kestrel, and AB would recommend:

Bryan Litz:




4dof does things a little different. One reason is, they have their own bullet data. But also they use methods like axial form factor. You can true something with just that, BUT if you true velocity within 600y, you will reduce how much axial form factor you use at greater distance potentially. The less you have to true the better.
 
dfanonymous said:
Accurate by who? It depends where you are having problems. BC has little to no effect on impacts within 600 for most cartridges.
BC starts to become noticeable with compounding error at around 800 or so. That's when you would tweak your BC. This is so you can have it lined up to extreme ranges.

Even if I'm just a stranger on the internet whose background you don't know, it's actually how kestrel, and AB would recommend:

Bryan Litz:




4dof does things a little different. One reason is, they have their own bullet data. But also they use methods like axial form factor. You can true something with just that, BUT if you true velocity within 600y, you will reduce how much axial form factor you use at greater distance potentially. The less you have to true the better.
Click to expand...

Hey brother, no offense intended in my comments. And I do not know your background, nor you mine. I have not intended to discount anything you have said.

As to the accurate by who, unless I misheard him, in BarbourCreek's attached video he stated truing by BC was more accurate; if you were asking for an actual name. But that is not the first time I have heard that stated in the last couple decades since I started hunting at longer distances. I do see Litz takes a different stance by your attached video. I'll try both in the future and let my data show me the way.

I like the 4DOF model has more variables and a custom curve. I am sure if I shot beyond 1000yds I would need a better app. I've never had the scenario that showed me my app's weaknesses like this. But my other chambers also shoot much flatter and likely hid error better since they are all at least 200fps faster than my 33SM.

Thanks for the links.
 
300 Driver said:
Hey brother, no offense intended in my comments. And I do not know your background, nor you mine. I have not intended to discount anything you have said.

As to the accurate by who, unless I misheard him, in BarbourCreek's attached video he stated truing by BC was more accurate; if you were asking for an actual name. But that is not the first time I have heard that stated in the last couple decades since I started hunting at longer distances. I do see Litz takes a different stance by your attached video. I'll try both in the future and let my data show me the way.

I like the 4DOF model has more variables and a custom curve. I am sure if I shot beyond 1000yds I would need a better app. I've never had the scenario that showed me my app's weaknesses like this. But my other chambers also shoot much flatter and likely hid error better since they are all at least 200fps faster than my 33SM.

Thanks for the links.
Click to expand...
No offense taken. I was just saying.
If I was annoyed, I usually just start trolling or butt out.

James has made other videos. More accurate is one opinion. Like I said, the BC effect is negligible at 500y. But if you can't get in the waterline at that distance it's pretty much always MV. Or shooter error that you true out in the MV.


si=W0a2DYf762NO-7Ug

There's hundreds of topics on this, and a plethora of good shooters that have it on record and in excessive detail. This thread will scratch the surface on how dumb this can get. As long as you get it to where you're happy though I guess it doesn't matter how you get there. Sort of. Until you try to go further. Or subsonic.
www.snipershide.com

Truing MV vs. BC

When truing MV at what yard marker do you start truing BC? and is that yard marker different for different calibers?
www.snipershide.com www.snipershide.com
 
Agree under 600 yards can do either Over 800 or near transonic definitely BC with real crono velocity
That's one of the reasons I made my app because trued velocity at LongRange was always iffy when you shoot closer or further than trued range.
 
Another thing that got me a couple of months ago. I was off on my scope height and it had my dope all messed up until I got it corrected.
 
RockyMtnMT said:
Another thing that got me a couple of months ago. I was off on my scope height and it had my dope all messed up until I got it corrected.
Click to expand...
Out of curiosity, what is your process to measure scope height?

My action is round, so I take half the diameter plus my rail height and height to top of bottom ring at the forward ring. When I use my calipers, I get a similar measurement but I don't use the caliper measurement because I can't precisely determine bore line.
 

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