Range Report A bit confused - real world drop compared to JBM

[CharlieTN]

I have a bit of a confusing situation on my hands.

My current target load is a Hornady 168 bthp over 43 grains of H4895 in a .308. Was averaging 2629 fps chronographed from a 700 SPS varmint with a 26' barrel. Recently had the barrel cut to 22" so I headed to the range again to check how much velocity I lost.

After running some over the chronograph I came up with an average of 2609 fps. Not too bad of a velocity loss for cutting 4" off the barrel. Groups were decent and I was quite happy. I decided to do a 200 yard drop (basically a "T" shaped target) to confirm the drop so I could compare it to the data from JBM ballistics. With the previous speed JBM gave me an estimated drop at 200 yards of 4.4" (.6 mils) and this held true with a drop target. The only problem is that this time with the slower velocity my drop was right at 3" at 200 yards. JBM says I should have a drop of 4.5".

I'm using Litz G7 BC of .222 which showed to be accurate with the previous speed.

One thought is that previously with the longer barrel most of my shooting had been at roughtly 60-90 degrees. The temp the day I chronographed this load in the shorter barrel was roughtly 40 degrees F. Could that make that kind of a difference? According to JBM it shouldn't.

Any thoughts or suggestions would be greatly welcome.

Charlie

 

[waste_knot]

Did you have your zero range set at 100 yrds in both cases.

Just to add a tidbit of data im getting 2620fps with 42.8 IMR 4895 22" Bartlein. H4895 is a bit faster so your numbers sound slow.

 

[jbmillard]

Without knowing ALL the inputs, it's pretty much impossible to tell what the problem is.

 

[roggom]

My guess is since you recently had your muzzle cut, you or the smith cleaned the barrel might well and have not fouled it yet. Lower temps usually contribute to lower DA, but at 200Y should be negligible, well not 1.5".If anything the POI should have gone down. Put another 50 -100 rounds through it and rechrony it.

 

[CharlieTN]

Did you have your zero range set at 100 yrds in both cases.

Yes I did. The target I use is a large "T". I maintain the 100yard zero and aim at the top intersection of the "T" at a given distance further then measure the actual drop from the POA.

 

[CharlieTN]

Trajectory (Simplified)
Input Data
Manufacturer: Hornady Description: HPBT (Litz)
Caliber: 0.308 in Weight: 168.0 gr
Ballistic Coefficient: 0.222 G7 (ICAO)

Muzzle Velocity: 2600.0 ft/s

Sight Height: 1.50 in Line Of Sight Angle: 0.0 deg
Cant Angle: 0.0 deg

Wind Speed: 10.0 mph Target Speed: 10.0 mph

Temperature: 59.0 °F Pressure: 29.92 in Hg
Humidity: 0 % Altitude: 0.0 ft

Std. Atmosphere at Altitude: No Pressure is Corrected: Yes
Zero at Max. Point Blank Range: No Target Relative Drops: Yes
Column 1 Units: 1.00 in Column 2 Units: 1.00 mil
Round Output to Whole Numbers: No
Output Data
Elevation: 4.022 MOA Windage: 0.000 MOA

Atmospheric Density: 0.07647 lb/ft³ Speed of Sound: 1116.4 ft/s

Maximum PBR: 313 yd Maximum PBR Zero: 266 yd
Range of Maximum Height: 147 yd Energy at Maximum PBR: 1498.1 ft•lbs

Sectional Density: 0.253 lb/in²

 

[Sniper Uncle]

It seems really odd to me that you only lost 20FPS by cutting 4" off the barrel--- shouldn't you be closer to 60 or more??? Dan Lilja told me that if i put a longer barrel on my 30.06, I could expect 20-25FPS gain per inch of added length.

One other thought, I haven't got hard data to prove this, but I have heard that if the air is humid, there can be significant retained velocity differences at distance from when the air is dry----I can't remember which way it goes, faster or slower with increased humidity. Where the humidity levels relatively the same both days?

Don't flame me if I haven't got a full grasp on these principles---I'm also trying to learn here, and see if I have got correct what I think I have learned so far....
Cheers,
Tim

 

[CharlieTN]

It seems really odd to me that you only lost 20FPS by cutting 4" off the barrel--- shouldn't you be closer to 60 or more??? Dan Lilja told me that if i put a longer barrel on my 30.06, I could expect 20-25FPS gain per inch of added length.

One other thought, I haven't got hard data to prove this, but I have heard that if the air is humid, there can be significant retained velocity differences at distance from when the air is dry----I can't remember which way it goes, faster or slower with increased humidity. Where the humidity levels relatively the same both days?

Don't flame me if I haven't got a full grasp on these principles---I'm also trying to learn here, and see if I have got correct what I think I have learned so far....
Cheers,
Tim

I intentionally used H4895 from the beginning knowing I was going to chop the barrel to 22" so I didn't expect a significant velocity loss.

One thing I did think of, not sure if this will make a difference, but my rifle is in a Bell & Carlson Medalist M40 which calls for 65in lbs of torque. I'm not sure if the smith did this or not. I'll check that and see where it goes.

Charlie

 

[jbailey]

Go out and redo this. You have not rewritten the laws of ballistics, which is a mature science and EVERY accurate. It is tough to shoot 1" at 200yds, so your results might simply be user error, such as a parallax adjustment. All the D.A. and BC stuff are nearly irrevelant at 200yds, so don't look for errors there. I would say, are you certain that your scope is 1.5" over the bore? It is possible, but most setups I see are 2.0".

Good luck!

 

[jbailey]

One other thought, I haven't got hard data to prove this, but I have heard that if the air is humid, there can be significant retained velocity differences at distance from when the air is dry----I can't remember which way it goes, faster or slower with increased humidity. Where the humidity levels relatively the same both days?

Don't flame me if I haven't got a full grasp on these principles---I'm also trying to learn here, and see if I have got correct what I think I have learned so far....
Cheers,
Tim

Tim-
No, your humidity suggestion is not a possible cause. First, humidity has the smallest impact on air density of the three major factors. Barometric pressure has the most, temp the second, and humidity is so far down that we can safely ignore it even if shooting to 1000yds. 0% to 100%, pretty much the same dope. Second, none of these environmental factors have much effect at 200yds. At least not the impact that the OP is seeing.

 

[Aimsmall55]

There are other factors to consider. Scope height , temp of ammo, your elevation. The angle which you were shooting.
I've always used jbm as a "starter kit " and usually run simulations on it. It will get you close but it's not dead on. If your input parameters were entered to suit your environmental conditions it would probably be more accurate. I don't think the g7 even matters at 200 yards. The g1 would be exactly the same most likely. G7 is best suited for popping stuff way out there and for vld bullets. But given your in Tn your altitude is probably 1000 asl?? If your from the mountain area even higher. What's your actual asl, temp which you were shooting??

 

[CharlieTN]

Go out and redo this. You have not rewritten the laws of ballistics, which is a mature science and EVERY accurate. It is tough to shoot 1" at 200yds, so your results might simply be user error, such as a parallax adjustment. All the D.A. and BC stuff are nearly irrevelant at 200yds, so don't look for errors there. I would say, are you certain that your scope is 1.5" over the bore? It is possible, but most setups I see are 2.0".

Good luck!

Parallax is certainly a possibility.

Let me back pedal a moment. I am new to this and certainly still learning. I have no doubt it is something I'm doing wrong or entering wrong. I'm not entirely certain the chrono results are the most accurate. The sun was at an acute angle and later in the session the data started to seem suspect. But again, I chopped 4" off the barrel so I would have thought it would have dropped more, not less.

Parallax is something I'm trying to become more aware of and did adjust that day. That could be the problem as well. My groups were consistently holding .8 moa on this particular day and I measured the drop to the center of the group. Not the best but decent.

I will be repeating this test in 2 weeks when I have time to get to the range again.

Everyone's input is greatly appreciated as it helps me to have areas to look at and to tweak.

Thank you,

Charlie

 

[Sniper Uncle]

Tim-
No, your humidity suggestion is not a possible cause. First, humidity has the smallest impact on air density of the three major factors. Barometric pressure has the most, temp the second, and humidity is so far down that we can safely ignore it even if shooting to 1000yds. 0% to 100%, pretty much the same dope. Second, none of these environmental factors have much effect at 200yds. At least not the impact that the OP is seeing.

Thank you for the kind correction. I haven't really studied these things yet, as I have been concentrating on my reloading and shooting skills before I get into the little peripheral details too much.
This one way I learn things, and I appreciate the help.

 

[CharlieTN]

Thank you all for the suggestions and thoughts.

Aimsmall55 - as you and others have mentioned, most of the factors that can change the POI will have little effect at 200 yards. That is why I was so surprised at the difference. The only thing I have changed is the length of the barrel.

I did go back and start checking things that you and others mentioned and found one factor that was off. Originally the rifle was set up with a 6-24x40 scope with a 1" tube and that setup measured 1.5" scope height. I didn't think to recheck this measurement when I switched to the Weaver 3-15x50 last year. The measurement now is just under 2".

That did change the drop calculated by JBM to closer to what I was seeing. I'm also going to go back in 2 weeks and re-run the chronograph and the drop tests.

Thanks again for all of the suggestions.

Charlie

 

[Michael Courtney]

Scope height or parallax are the most likely errors to cause disagreement with JBM. If you get the inputs right, JBM is money.

 

[CharlieTN]

Scope height or parallax are the most likely errors to cause disagreement with JBM. If you get the inputs right, JBM is money.

Thanks for the input. I did re-measure the scope and was off by 1/2" so that made a bit of a difference in JBM. Parallax is very likely an issue and I have just begun to play around with adjusting it. I've got a date at the range again next Saturday to check the chrono again and will try it again.

Charlie

 

[cmh2007]

has anybody considered that the barrel harmonics may have changed and being it is now 22 inch the barrel may be releasing the bullet on the up swing of the harmonic wave?

 

[jbailey]

has anybody considered that the barrel harmonics may have changed and being it is now 22 inch the barrel may be releasing the bullet on the up swing of the harmonic wave?

That would impact his 100yd zero, but once that is squared away, it would have nothing more to do with the ballistic solution.

 

[CapDog]

Chrono graphs are finicky. Get solid on your scope height and environmental inputs then take it to distance. At least 500 and preferably further.Then back check it and get an accurate velocity. Once you do that you will probably find the dial ups are pretty close to being bang on.

 

[Myanmar Shave]

This is probably not the op's issue, but be aware. I have been shooting 1000 yds a little, and my elevation is way off from JBM calculations. Is just dawned on me that since my scope is metric, I should have my parallax set to 914 (meters) instead of 1000 (yds).

 

[Lowlight]

There is a process to using a ballistic calculator and if you don't follow the process you cannot expect it to work.

1. You have to Calibrate your Scope up to 100% of the useable travel.

That means you have to test tracking, mostly your elevation adjustment but also your windage to see if it affects your elevation. Most scopes are the weak link in the system.

You have to shoot your rifle using the "Try dope" (this is the initial information JBM or any software gives you to start) This should be done at as many ranges as possible, but 300 and 800 yards as a minimum. Then you can begin to true your software.

The Chronograph is key, you need as much real time data as possible when truing. I would recommend shooting your groups at 300 and 800 over your Chronograph. (Not all Chronographs are created equal you should expect them to be off before the software.)

As someone noted, we have a ton of data especially on a 308, if you are off more than 1 MOA you can count on two things first, Operator Error in Inputting or Scope Error ... those are the first two places to look. 1 MOA is generally as close as you are gonna get, as the software is just a starting point and not set in stone. (JBM Simple Trajectory does not use Barrel Twist nor can it predict variations in Bullet Weight, Powder, etc... It only knows what you tell it. If you tell it something wrong the output will be wrong. Most of these variations will show up at and after 500 yards as you have to compound the error enough which is what distance does... if you are seeing errors at 200 yards that don't put you within an Inch, you have to look to Operator Error or Scope Tracking.

Software requires effort, if you ever take the XLR Class @ Gunsite they spend two days calibrating the system, that is shooter, scope, software... Shooter just as much because all software lacks the most important input function of Long Distance Shooting, the Shooter. It cannot predict when you have poor fundamentals. It believes you are consistent shot after shot executing the firing task perfect. We all know that doesn't happen. you can easily be a guy with a flinch or trigger crush who cause his rounds to go low, or worse.