Is velocity really my issue? Or something else?

[cbh560]

I have been loading for my 7-08AI and have settled on a load with 140 Nosler BT with 44.6gr of Varget at a velocity of 2925 fps as measured by my magnetospeed. Today I was out shooting from 200 to 500 yards to confirm my dope. Groups were tight with my best group (3-shot) at 0.3MOA at 500 yards.

I was basing my drop from calculated drop chart using the Strelok app. Actual drop was close to calculated but not quite right, being off by .25 to .5 MOA from 300 to 500 yards, respectively However, when I drop the velocity in the calculation to 2875 fps, they match almost perfectly.

I trust my magnetospeed to be providing accurate velocity. The 2925 fps was an average of about 15 shots. ES over those 15 was approximately 30 fps.

Is it likely that my velocity is in fact incorrect? Or is there another variable in the drop calculations that might make up for the difference between calculated and actual drops?

 

[VancouverS4]

Does the app take altitude, humidity, and other atmospherics into consideration? If so make sure it's set right for the zero atmosphere and the atmosphere of the day/place you're shooting.

 

[ruby76]

Could be scope clicks not exactly .25 moa, or bullet ballistic coefficient.

 

[cbh560]

Does the app take altitude, humidity, and other atmospherics into consideration? If so make sure it's set right for the zero atmosphere and the atmosphere of the day/place you're shooting.

It allows input for both zeroing weather as well as current weather. I input today's weather for zeroing ( I re-zeroed today) but hadn't changed the current weather input. However when I change the current to what it should have been it actually makes the difference between calculated and actual drop a little bigger

 

[cbh560]

Could be scope clicks not exactly .25 moa, or bullet ballistic coefficient.

I've done a box test (it was only at 100 yards) and clicks were good on the scope. Maybe I should repeat at greater distance to confirm. If it was BC, I would have to drop it from the Nosler's value of .485 to .415 to make up the difference. Seems like too big of a difference off Nosler's value to be correct.

 

[Steve.693]

Manufacturers often give you the best number they can for marketing.

BC is also changed by velocity so that can be a factor. I haven't used strelok but if you can input the g7 bc it seems to be better for modern rifle bullets than the g1 model.

JBM ballistics is a good source for large bullet library and a free calculator as well.

 

[emerson]

Manufacturers often give you the best number they can for marketing.

BC is also changed by velocity so that can be a factor. I haven't used strelok but if you can input the g7 bc it seems to be better for modern rifle bullets than the g1 model.

JBM ballistics is a good source for large bullet library and a free calculator as well.

This. I’m having the same issue with 22LR out to 325M. I believe it’s my ballistic coefficient that is needs to be trued to my drops. Also the G7 coefficient is a better representation of many boat tail ish hunting bullets.

 

[BCBRAD]

With the availability of tools such as accurate chronographs /radars and exterior ballistic programs the BC of some well advertised bullets are a little lacking in the accuracy of the stated BC's.

To be fair , the rate of twist you have may reduce some of the effective BC ,the manufacturer's testing conditions of the bullet you are using may not be the same, this leaves the opportunity , along with an exterior ballistic information, to true up the BC to actual conditions that you find yourself
in.

 

[Potashminer]

I have Sierra Loading Manual #5 - Copywrite is 2003. They list multiple different G1 B.C. values for their bullets - for example, their 250 grain 338 SPBT has four values listed - for speeds below 1,600 fps on up to above 2,500 fps - from .537 to .565. The number gets bigger as bullet travels slower. If their 7mm 140 grain SPBT are similar to bullets you used, they show 3 values - .416 above 2,400 fps up to .423 below 1,800 fps. For your load and the distances that you shot, you might be crossing into all those velocity ranges?

 

[cbh560]

I have Sierra Loading Manual #5 - Copywrite is 2003. They list multiple different G1 B.C. values for their bullets - for example, their 250 grain 338 SPBT has four values listed - for speeds below 1,600 fps on up to above 2,500 fps - from .537 to .565. The number gets bigger as bullet travels slower. If their 7mm 140 grain SPBT are similar to bullets you used, they show 3 values - .416 above 2,400 fps up to .423 below 1,800 fps. For your load and the distances that you shot, you might be crossing into all those velocity ranges?

This is interesting. The calculator says that I would be at approximately 1960 fps at 500 yards. So it's possible that multiple BC values could be applicable during the bullet fight time. However, your example is a difference of .007 and in my case it's .700 (100x the value of your example)

 

[Potashminer]

I just do not know enough about it. Earlier manuals that I have do not even list B.C. - Speer #5. If your magnetospeed is like mine, there is no "lab certification" about it's accuracy - it does spit out a number, but I do not know how correct that is, or how many digits are correct. I would not put it past bullet maker's sales people to "stretch" a B.C. number to make it more "saleable". But your holes in target are real - that is what I think I would be relying on - the rest could be at least part "make believe"?

 

[cbh560]

I just do not know enough about it. Earlier manuals that I have do not even list B.C. - Speer #5. If your magnetospeed is like mine, there is no "lab certification" about it's accuracy - it does spit out a number, but I do not know how correct that is, or how many digits are correct. I would not put it past bullet maker's sales people to "stretch" a B.C. number to make it more "saleable". But your holes in target are real - that is what I think I would be relying on - the rest could be at least part "make believe"?

Thats a good point about about the chrono being calibrated. In this case I believe it's numbers as the reputable gentleman I got it from replaced it with a Labradar and told me all the magnetospeed readings were within 2 or 3 fps. Its also fairly consistent with a previous chronograph of mine.

I don't doubt manufacturers stretch the BC value for marketing purposes. Like you say, holes on target are real and reproducible, so ultimately that is the data that l will trust. But I tend to have an inquisitive mind and can't help asking "but why?"

 

[Terry Perkins]

Did you input the correct scope height? A half inch difference in measurement at 500 yards is worth .4 moa.

 

[Chilly807]

I'm of the belief that the numbers from any ballistics calculator are enough to get you in the ballpark, and may be very close in some cases. That's not to say they'll be perfect.

I've used the free Lapua calculator for a while now. It's quite good but I had to tweak the velocity input to match up to my actual drops at longer ranges. That's probably a result of chronograph error.
My chronograph probably isn't as good as either a Labradar or a Magnetospeed, but again, it's close enough to get me on target.
At 100-200 yds, a 40 fps error isn't really noticeable, but as you stretch it out past 600 yds it becomes more apparent.
Like your results, my predicted drops at various ranges are usually within 1 MOA of actual scope settings. When you consider the number of variables in the equation that's pretty good.

 

[cbh560]

Did you input the correct scope height? A half inch difference in measurement at 500 yards is worth .4 moa.

Scope height is input correct at 1.49". That measurement was taken from center of the bore to center of objective lens. However I will double check it.

 

[jethunter]

You"re overthinking it. Holes in the target don't lie. BC is not a constant, it is average for a specific velocity range. You could pay to have the magnetospeed calibrated but the target already told you everything you need to know.

 

[cbh560]

You"re overthinking it. Holes in the target don't lie. BC is not a constant, it is average for a specific velocity range. You could pay to have the magnetospeed calibrated but the target already told you everything you need to know.

Agreed, likely overthinking it but there is a reason for that. The scope on the rifle is a Leupold with the CDS dial. I am confirming drop to make sure that the info I give to Leupold to engrave my new CDS dial is in fact correct. I don't want them punching in the velocity, BC from the bullet manufacturer, etc. into a drop calculator and then have them engrave the dial to the "wrong" calculated specs. So I'm working to understand if any of the other variables are the root cause of my issue. I'd hate to take a shot at 500-700 yards only to learn that my new dial is a full 1 MOA out from actual value.

 

[jethunter]

Agreed, likely overthinking it but there is a reason for that. The scope on the rifle is a Leupold with the CDS dial. I am confirming drop to make sure that the info I give to Leupold to engrave my new CDS dial is in fact correct. I don't want them punching in the velocity, BC from the bullet manufacturer, etc. into a drop calculator and then have them engrave the dial to the "wrong" calculated specs. So I'm working to understand if any of the other variables are the root cause of my issue. I'd hate to take a shot at 500-700 yards only to learn that my new dial is a full 1 MOA out from actual value.

What info does Leuopold need to calibrate the dial? Wouldn't a drop table be sufficient? or do they need a BC?

 

[cbh560]

What info does Leuopold need to calibrate the dial? Wouldn't a drop table be sufficient? or do they need a BC?

All the usual drop calculator variables. Bullet weight, muzzle velocity, avg temp, avg altitude, calibre, zero distance, etc.

Its actually Korth Group that will be fulfilling the order. I've sent them the question if they will accept my real world drop values, but do not yet have a reply.

 

[Terry Perkins]

Scope height is input correct at 1.49". That measurement was taken from center of the bore to center of objective lens. However I will double check it.

So, I checked my numbers and my scope. My numbers (at 321 yards) show a increased drop of .2moa when measuring the distance between the center of the bore and the center of the objective lens. I measure from the center of the bore to the center of the ocular lens. This matches the numbers I input into my program and the results on paper.