Adjusting Muzzle Velocity to Correct Ballistics Chart

[pwrdbycotn]

I was shooting my 300RUM today and testing a new load. I was shooting 93gr Retumbo, using 210 Bergers, and getting a MV of 3123fps using a Magnetospeed V2. I shot at 100, 400 and 840 yards. At 400, the Shooter app and G7BR2 were calling for 5.2 moa up. It actually only took 4.125 (1/8 moa turrets). At 840, they were calling for 17 up and it actually only took 15.375. In order to get my charts to show the correct drops, I have to increase the MV to 3275fps. Is it normal to have to increase velocity that much to make the charts match real-world data?

 

[7magcreedmoor]

First question is have you tested your scope's tracking? If the turret adjustments are accurate, then the second question is do you adjust BC or do you adjust MV? Berger's BCs are regarded as pretty rock solid, so I would choose to adjust MV. Your impacts are higher than what your program predicted, so that means the bullet must be going faster. Whatever MV number it takes to make things match must therefore be your true MV. When I do load development, I always take my chrono numbers with a grain of salt. When we consider how the device is attempting to measure the speed of an object going that fast, over such a short distance, it's pretty amazing we get any useful information out of the deal. Shooting at multiple longer distances with a solid short range zero and comparing the outcome is always my final determination for velocity. Sounds like you got a good load there.

 

[pwrdbycotn]

First question is have you tested your scope's tracking? If the turret adjustments are accurate, then the second question is do you adjust BC or do you adjust MV? Berger's BCs are regarded as pretty rock solid, so I would choose to adjust MV. Your impacts are higher than what your program predicted, so that means the bullet must be going faster. Whatever MV number it takes to make things match must therefore be your true MV. When I do load development, I always take my chrono numbers with a grain of salt. When we consider how the device is attempting to measure the speed of an object going that fast, over such a short distance, it's pretty amazing we get any useful information out of the deal. Shooting at multiple longer distances with a solid short range zero and comparing the outcome is always my final determination for velocity. Sounds like you got a good load there.

Tracking is dead on! It's a March 5-32 and I tested the turrets, both elevation and windage, with great results. The reason I went with increasing MV is exactly because of what you stated; Berger's B.C.'s are well-known to be correct. I really didn't expect the velocity to be this high, however. I'm certainly not complaining, though. It was shooting 1.25" groups at 400 so I'll take it just like it is.

 

[bill123]

Have you confirmed that all of the other inputs in the app are correct? That seems like an awful large variance for a Magnetospeed. Do you have another rifle/ammo combo for which you are certain of the MV? If so, I'd check the Magnetospeed against that, just to make sure the chrono is accurate.

 

[pwrdbycotn]

Have you confirmed that all of the other inputs in the app are correct? That seems like an awful large variance for a Magnetospeed. Do you have another rifle/ammo combo for which you are certain of the MV? If so, I'd check the Magnetospeed against that, just to make sure the chrono is accurate.

Yes, all atmospheric and other variables were confirmed. As luck would have it, I sold my 6.5x47 back in April and currently have a 28 Nosler and 6.5 saum 4s build at the gunsmith right now. I can/will check it against my 223 ar-15 in the coming days, but that's all I have to check against. After a bit of research from older threads here, 3275 fps isn't really that far off for the load considering a 27" barrel and atmospheric conditions; low humidity, 61*F.
I was shooting at 840 yds yesterday after sighting and chrono'ing at 100 and kept missing. I was using the recommended drops and my spotter kept telling me I was high. It's hard to tell exactly how much due to shooting on flat ground over a picked cotton field. However, he kept seeing cotton fly up in the distance in the next field well beyond the target. I ran out of ammo before we could get it figured out. Today I decided to back up to 400 yards and check the recommended drops. I hit within 1" of the top of the target. That's when we figured out something was off. I adjusted the scope the required amount and hit dead on within 1/2". I shot three more shots; adjusting one minute up and one right, then two minutes down and two left, then back to bulleye at 400 yds. I wanted to confirm the tracking of the scope and these shots confirmed that. Backing up to 840 again, I dialed 16 up instead of the called-for 17 and low-and-behold I was slightly over 8" high. Backed it down to 15 minutes and was slightly low. Came back to 15.375 and shot a little over a 3" group all around center. So with the 400 yard called-for drops and actual drops being just about a minute apart and the 800 yard called for drops and actual drops being just over 1.5 minutes apart I believe the velocity was off.

 

[bill123]

Sounds like you have it under control. You're going to have a blast with the 6.5 SAUM.

 

[Bart B]

All ballistic software does not give the same answers for a given bullet and environment.

Does anyone ever consider this to be why their bullets paths don't match what the software says?

 

[pwrdbycotn]

All ballistic software does not give the same answers for a given bullet and environment.

Does anyone ever consider this to be why their bullets paths don't match what the software says?

I believe that most everyone considers this. Otherwise all that would be required is sighting in and chronographing a load and then trusting the software output to be correct at any given yardage. Anyone with even mild common sense will test their loads over multiple ranges and adjust the variables on the software accordingly to match the true bullet path.

 

[Bart B]

Anyone with even mild common sense will test their loads over multiple ranges and adjust the variables on the software accordingly to match the true bullet path.

How do they verify their chronograph is correct?

 

[pwrdbycotn]

How do they verify their chronograph is correct?

Hmm, well a couple thoughts and questions come to mind.

1. Why does the chrono have to be 100% correct? Isn't the only thing that is required to be correct is the known bullet path? Since this is a hunting website I feel as though this is the only thing that truly matters since we are shooting at live animals and not just paper.
2. Can't the chronograph numbers be adjusted easily based on that known bullet path? If the numbers can be easily adjusted, then isn't a chronograph just a tool that is used to give a good starting point for making scope adjustments at various ranges?
3. Is there a chronograph made that is guaranteed to be 100% correct, 100% of the time? If so, is it remotely affordable and where can it be found?
4. Asking me about how other people verify their chronograph is correct doesn't really belong in this thread. I started this thread asking about adjusting muzzle velocity changes in ballistics calculators to match verified bullet paths vs possibly changing B.C's to achieve the correct drop. I was also surprised that the numbers had to be changed so much to get it correct. It took me all of 10 seconds to change those numbers so I'm certainly not laying awake at night fretting over whether my chronograph is correct or whether the software used is giving me the correct drops. I have better things to do and more important things on my mind than to worry over useless drivel like that when it takes all of 10 seconds to make it right.
5. Apparently the adjustments made to the muzzle velocity were the correct thing to do because tonight my G7BR2 called for 6.6 minutes up and I dropped a whitetail in his tracks at 506 yards.

 

[7magcreedmoor]

I managed to get to the range today for some trajectory validation. My Chrony was showing an average of 2750 fps, but to get the drops to match at 400 and 700 yards I had to bump the vel up to 2875 fps. As I said before, I only use the chrony for a comparison between loads, not the final authority on true velocity. The proof is in the shooting. After I have 200 rounds through this barrel, I will consider it fully broken in and by then will have this batch of brass all once-fired. At that point I will test again with a ladder in the fired brass for final tweaking of the load.

 

[Wedgy]

The bullet library in Applied Ballistics lists them with their adjusted BC, for instance the Barnes 6.5mm 127 grain LRX claims .468 but is listed as .446 so that's a good start.
When I run the Magneto Speed and the CED at the same time the MS gives 20-30fps higher due to the distance from the barrel of the CED but they are always close to each others readings. So like the OP, I am hesitant to adjust MV up to meet the known drops when all the other inputs appear to be correct. Inevitably my bullets seem to grow wings, especially after 600 yards.

 

[Scot E]

How do they verify their chronograph is correct?

Most ballistic apps have trajectory calibration tools that eliminates any error you may find in a chrony.

Last year I went to help a buddy gather drop numbers for his new load and forgot my chrony. Instead of calling it a wasted day we estimated what we thought the fps would be based off of other guns and our experience with them, then calibrated it at 600 and 750 yards. We were off by 75 fps, didn't subtract enough for his shorter barrel, but one fix in the ballistic app and we were golden.

Most guys are going to be verifying their drops at distance anyway so plugging in those drop numbers to get corrected velocity is easy, simple, fast, and very accurate.

If you are running spin drift, Coriolis, and atmospheric conditions and you trajectory validate, every ballistic app I have used is very close to the same for outputs in my experience.