Verifying Muzzle Velocity

[GunnyUSMC]

I have two rifles i am preparing for my first ELR-Light match. One is a 300PRC and the other is a 6.5 Creedmoor.

First - I developed loads that provide a good MV with nice sub-MOA groups using a 100yd Zero
Second - Compared my actual elevation at 800yds to what my Kestrel said it should be

Here is where I ran into a dilemma. The Kestrel data and actual data for my 6.5 creedmoor was only off by about .2 MOA so this wasnt too bad (I am using a Vortex Golden Eagle on this setup). However, the Kestrel data and actual data for my 300PRC was off by appx 1 MIL (I am using a Vortex Raxor HD Gen III 6x36).

Using the Kestrel, i corrected both MVs then tested my elevation adjustments at 500 and 600yds with excellent results.

So here is where I have a question/concern. How can the Elevation predicted by my Kestrel be so far off of my actual elevation? I double checked everything in my kestrel and all looks well. My actual MV for the 300PRC was an average of 2911 but the corrected MV on my kestrel was 2789. What is even more puzzling is that I did a sanity check with JBM Ballistics and found that it predicted a much closer drop to my actual than the kestrel.

Confused ... looking for answers. Could this be accounted for in the Scope itself? I havent done a tall target test on the Razor yet because it is brand new.

 

[Malum Prohibitum]

In for answers . . .

 

[Rio Precision Gunworks]

What bullet?

It's likely some sort of input issue.

 

[GunnyUSMC]

for my 300PRC I am using the Berger 230gr OTM Hybrid

 

[Aftermath]

Check the input parameters. Scope height above bore is my first suspect.

 

[Dogtown]

At that short of a distance, it’s clearly not a BC issue. Your measured MV isn’t too far off from adjusted value, so maybe borrow a friend’s chrono to see if it measures a similar value? I’m not sure a Height Over Bore tweak would make that much of a difference at those distances, unless you input one that was way off.

 

[GunnyUSMC]

I will check the scope above bore again. I am running a eratac adjustable inclination mount that is pretty tall. My measurement of scope over bore was 2.9".

As for the MV, I originally got my MV using a LabRadar. I just purchased the new Garmin Xero and used it for the first time last week. The measured MV from both was pretty close.

here is where i got confused though is i entered the same values in both the kestrel and JBM with significantly different results

 

[Dogtown]

I would assume same DA in both? Because that’s really weird if all the variables are the same but you’re getting a mismatch.

Side note: isn’t that Garmin awesome?

 

[GunnyUSMC]

Side note: isn’t that Garmin awesome?

yea

i have never had a significant problem with my LabRadar other than getting shoved aside from my muzzle blast just enough to not track.

With the Garmin, just set it and forget it and without any additional steps, my phone app is kept updated automatically. I just set it under my muzzle and shoot. eazy peazy

 

[ShtrRdy]

On the Kestrel set up are you using powder temperature variation correction? Or "zero atmosphere" correction?

 

[GunnyUSMC]

On the Kestrel set up are you using powder temperature variation correction? Or "zero atmosphere" correction?

I might need a little guidance here. MV Temp is off. I am not able to figure out where to check the specific parameters you mention here

 

[Maurygold]

Height over bore. It’s always height over bore

 

[GunnyUSMC]

How would you measure height over bore if you have an incline mount?

One end would be higher over the bore than the other

I measured the middle of a side turrent (paralax or windage) to the center of the bore

 

[Maurygold]

the general consensus is to measure at the turret. middle of the tube to the middle of the barrel bore.

 

[GunnyUSMC]

the general consensus is to measure at the turret. middle of the tube to the middle of the barrel bore.

Roger that...

That is what i figured

thanks

 

[Maurygold]

if HOB is good and you know velocity what bullet and BC are you using

 

[M4orturnate]

Based on the optic you're running I wouldn't count out some errors % with each click. A lot of times it doesn't even start to show up until you're in the second revolution

 

[M4orturnate]

How would you measure height over bore if you have an incline mount?

One end would be higher over the bore than the other

I measured the middle of a side turrent (paralax or windage) to the center of the bore

Center of tube to center of barrel. Incline of scope won't make a difference

 

[Maurygold]

Center of tube to center of barrel. Incline of scope won't make a difference

that measurement is different at the eye piece and at the front glass if you're scope is on incline so yes it does matter and the answer is generally use the middle as that is where the movement occurs.

 

[ShtrRdy]

I might need a little guidance here. MV Temp is off. I am not able to figure out where to check the specific parameters you mention here

I don't have a Kestrel with a ballistic app so maybe my suggestion doesn't apply. I use the "Shooter" app on an Android phone . Within the set up dialogue for "ammunition" is a box labeled "Enable Zero Atmosphere". I don't know if it makes much difference.

 

[M4orturnate]

that measurement is different at the eye piece and at the front glass if you're scope is on incline so yes it does matter and the answer is generally use the middle as that is where the movement occurs.

It won't matter I promise. If you want to be anal use the center of the windage turret to center of the bore

 

[Maurygold]

I don't have a Kestrel with a ballistic app so maybe my suggestion doesn't apply. I use the "Shooter" app on an Android phone . Within the set up dialogue for "ammunition" is a box labeled "Enable Zero Atmosphere". I don't know if it makes much difference.

yeah, that's not how kestrels work. @GunnyUSMC, you don't need mv temp on if you have the real velocity that day - that only adjusts the mv based on temp tables you build. in the long run, you should use it, but that's not creating the issue here.

 

[Maurygold]

It won't matter I promise. If you want to be anal use the center of the windage turret to center of the bore

you mean exactly like we already said? cool.

 

[secondofangle2]

you gotta recheck all variables for accuracy. DOn't correct shit with the Kestrel till you find the error. FIrst thing I would do is input the same variables into another calculator and see if they match up.

At 800 yards with a 300 PRC, a lot of errors are going to be too small to matter. You're dialing up, what, 4-5 MRAD? If you're 1MRAD off (you did say one MIL/MRAD, correct?), that's a 20-25% error. Here are things that would NOT account for that, even if they all lined up:

• MV if measured with a radar device
• scope tracking error
• scope height over bore error
• ranging to target error up to about 75 yards to target
• 5000 ft DA error
• slope to target
• small BC variations

Here are a couple of things that COULD account for an error that large:

• a nonsensical entry such as DA 30000 from flying on a plane with your kestrel or a 10" height over bore
• a G1/G7 BC entry error
• fps vs m/s or other Units of measurement input errors
• A loose scope mount
• A wandering or other zero error
• Shooter error

 

[bax]

I am shooting a 300 PRC with Berger 230 grain hybrids, 26 inch barrel, estimating 2725 fps. I use the Applied Ballistics app on my phone. I select the bullet from the bullet library. I set altitude to 0 in Applied Ballistics, enter the pressure from my kestrel, and select "pressure is absolute". I use a kestrel where the altitude is set to zero feet that way I get absolute pressure, not pressure adjusted to sea level. I enter temperature and humidity. I am usually within 0.2 mils at 1000, sometimes 0.1. Humidity doesn't matter much, both temperature and pressure matter a lot. I find that height-over-bore doesn't matter - change it to 1 inch and compute then 3 inches and compute. If nothing else changes, I see almost no effect. Right now, using standard atmosphere and pressure is absolute, AB says U5.5. That sounds roughly right to me. As I shot thru the winter, I was getting about 7.2 at 1k most of the time.

 

[GunnyUSMC]

I am shooting a 300 PRC with Berger 230 grain hybrids, 26 inch barrel, estimating 2725 fps. I use the Applied Ballistics app on my phone. I select the bullet from the bullet library. I set altitude to 0 in Applied Ballistics, enter the pressure from my kestrel, and select "pressure is absolute". I use a kestrel where the altitude is set to zero feet that way I get absolute pressure, not pressure adjusted to sea level. I enter temperature and humidity. I am usually within 0.2 mils at 1000, sometimes 0.1. Humidity doesn't matter much, both temperature and pressure matter a lot. I find that height-over-bore doesn't matter - change it to 1 inch and compute then 3 inches and compute. If nothing else changes, I see almost no effect. Right now, using standard atmosphere and pressure is absolute, AB says U5.5. That sounds roughly right to me. As I shot thru the winter, I was getting about 7.2 at 1k most of the time.

Thanks ... that makes sense.

I just got back from my first ELR with my new gun. On Friday, I checked my zero where the match was being held and then checked my MV at 1000. Because I wasnt 100% confident in my MV to begin with, i went ahead and let the kestrel make the MV correction. What was interesting is that my MV seemed much closer to that which i calcaluated in the very beginning. I could have had some bad user data that was throwing me off. The big difference here is the elevation. We were high in the mountains of western VA.

The ELR light match was on Saturday and I calculated my drops based on this latest corrected MV. With this new shooting solution, I was able to get impacts on all targets with the best results coming from the last target at 1906 yards. I still have a lot of work to do but at least I was not totally embarassed.

 

[bax]

There are two primary factors determining drop: atmospheric drag on the bullet and gravity. As air density increases, bullets lose velocity faster so it takes longer to get to a given target. More time means that gravity has longer to work on the bullet and, for a given point of aim, the bullet will hit lower on the target.

Also cold air is more dense than warm air and sea level air is more dense than the air at the top of Spruce Knob, West Virginia. There is a way to estimate air pressure. Suppose that you start at sea level and rise straight up while measuring air pressure - think balloon. The pressure lapse rate, that is the amount of pressure change as the elevation changes, is about one inch of mercury per one thousand feet of elevation. The 1962 standard atmosphere is 29.92 inches of mercury (InHg) at 59 degrees F. Assuming a base of sea level and standard atmosphere and that Spruce Knob is about 4,500 feet, the absolute pressure would decrease from 29.92 InHg to about 25.42 InHg (29.92 minus 4.5 = 25.42) - that is, the air is less dense. As altitude increases, the air becomes less dense so temperature drops - about 3.6 degrees F per thousand feet. In the standard atmosphere, sea level temperature is 59 F so the temperature at the top of Spruce Knob would be about 42.8 F (59 F - (3.6 * 4.5) = 42.8 F).

For what it's worth, as altitude increases, the speed of sound also drops. Think a bit about what that does to the relative position of the transonic zone.

My devices don't seem to be happy unless I use station pressure and temperature (the pressure and temperature where I am). I don't use barometer or temperature adjusted to sea level, I am not at sea level. I tell my devices that I am at sea level and they give me good numbers.