Let's argue about BC's

[eddybo]

Groper, I shoot many rifles, I only have this problem with one rifle, so I dunno. I know how to adjust my click values and have had to do the same for several scopes. The repaired scope may actually be on another rifle. I do know that I often measure the distance between POA and POI and use the dials l to correct these and have never noticed any real deviations. I have also used the MOA reticule as part of the equation without noticing any real differences. Say one day I dial 12 MOA, then the next day instead of dialing hold 10 MOA and dial two MOA, no problems.

The only difference with this particular rifle is a 40MOA base rather than a 20MOA base. So I have to keep coming back to that in the back of my mind. I cannot wrap my mind around what relating to the base would cause this, but do not know enough about the subject to even venture a guess. I have not shot the rifle much in the last year because of illness, and probably will not play with it again until I start getting ready to head to NM to shoot cow elk.

I am not aware of a G7 BC for this bullet but would be interested in trying it.

Why would I change a known value such as velocity to make a correction to BC. Maybe you are finding problems with BC that you are factoring out in the same manner in which I resolve mine. I have a chart around somewhere with my actual drops shot on a tall target (10 ft). I can make these drops match exbal predictions almost perfectly out to a mile using a stepped G1 BC.

Most times when I build a rifle I have the rifle sighted in within 3 shots at 100, assuming I do a good job bore sighting the rifle by looking through the bore. Even cleaning between each round shost no.4 through the end of break in and load work is shot on my 800 yard target. (It is actually 787 but I account for that) In most instances I can take quickload estimates as a velocity plugged into exbal and be pretty close to where I need to be. I could change velocities instead of BCs but why would I. Velocity is a much easier measurement to verify.

Now when I arrive where ever I am going to hunt is a different story. By that time I have a "working BC" that I trust and have to hope that velocity and enviromental changes can be the only causes for POA-POI differences.

Eddy, my NXS 5.5-22x56 has +2.5% error - which means when i dial 40clicks, im really getting 41 on the target... Ive had other scopes with upto 5% error like this.

I shoot a group @ 100, dial 100clicks up, shoot another group then measure on the target group 1 center to group 2 center. I dont bother doing the same with windage because the most wind ive ever had to dial is about 20 clicks and a few percent of this is gonna be within 1 click anyway...

I use a "custom click value" in my ballistics calculator to take this error out in my trajectory calculation- which becomes (0.1mrad*1.025) clicks. For this scope.

Eddy, have you tried adjusting your velocity instead of the BC? I dont use my chrony anymore in this regard, because IT was the source of half my problems... If you have a bullet that Litz has tested, use the exact Litz G7 derived BC, make sure all other variables are correct, including your dead on zero @ 100 for that load, ignore your chrony and adjust the VELOCITY in your calc, and see how your trajectory fits then- i bet it you will hit everything at all distances out to 1 mile...

 

[groper]

Now when I arrive where ever I am going to hunt is a different story. By that time I have a "working BC" that I trust and have to hope that velocity and enviromental changes can be the only causes for POA-POI differences.

^^^ That there is reason its more important to have an accurate BC as opposed to any other variable. A bullet only has 1 correct and true BC at that velocity... the physics are not flexible and manipulating this BC only creates errors elsewhere in your calculations. Sometimes the errors are so so small you dont notice them and wont effect an accurate shot placement regardless... in other circumstances these errors can be absolutely critical.

Getting the BC wrong, will change the way the calculator adjusts your trajectory for different environmental conditions and it will induce more error than any other variable used in your calc in this regard. When your out hunting and you ready to take a shot, if you input environmental conditions with small errors such as a couple hundred feet altitude, or a few degrees temperature error etc into your calculator, it makes the shot less accurate but wont alter your calculated dope "all that much"... however if your trajectory has been developed prior to this hunting trip with a significant BC error (and by definition you must have had other errors or your trajectory wouldnt have fit where you tested it), it can cause a LARGE error in your dope when you go hunt/shoot in very different lands with greatly different elevations or very hot/cold temps etc... The problem is much more noticeable with smaller calibers and lower BC`s than with flat shooting magnums and high BC bullets, and it gets worse as the distance increases...

When i was less experienced, making mistakes like this cost me a 850yd kill with my little .308win when i went for a hunt @ 3000ft lower elevation and much colder temps than where i did my testing. Using 155gr bullets, with a quoted G1 BC of around .5xx from memory, the 850yd shot was a long shot for this rifle, so everything had to be perfect, and it wasnt... that shot, really annoyed me and it prompted me to much more careful with what i was doing. Had i known Litz derived G7 BC of 0.236 at that time, i ran the numbers again for the conditions i was in with my new numbers, my poor shot would have been dead on and my trajectory now matches in all conditions....

 

[Topshot]

A couple of things that I have picked up when validating loads.

1, use Litz measured G7 B.C. values where possible and take the manufacturers advertised B.C. with a grain of salt.

2, Calibrate your scope turret adjustment for its inbuilt error.

3, Dont shoot anywhere near trees if there is any wind. They induce down drafts that will drive you crazy.

4, Take your time getting proper atmospherics before each shot, don't rush this. The instrument needs time to stabilize and must be calibrated!

4, Use "Shooter" for Android smart phones. It has a great scope error factor feature.

5, Take a cheap chronograph reading as a guide only. They have error too.

Just a few tips, I am sure there are others.

 

[paphil]

What is the reason for knowing the exact BC ? When we use the BC and chronographed velocities to attempt to produce an accurate drop chart, we setting ourselves up for grief ! Manufactures BC's are nice and help us to get "on the paper" but don't address the variations of each gun. I just spent ten days at the eastern sports show trying to explain how to set up scopes for Best of the West and it seems that most shooters want an accurate drop chart but have bought the idea of the generic turret as being good enough. To get an extreamly accurate drop chart, several steps are necessary.
I use the program at the BOTW website with it's hugh list of bullets and BCs. Find your bullet and BC and begin entering your data. Scope height, click value, tempature, elevation, ect. Calculate to get a starting point and print out the chart to have as a range card.Now go shoot your data. I shoot in dead on at 200 and use the chart to find an on paper hold for 500 and 1000. The chart might suggest 64 clicks at 1000 and my gun might take 68 or 60. Write down your results. Do the same at 500 and as many other distances as you care to shoot. What we are doing is defining an arc for your bullet. Take these results back to the computer program and plug in the far target information and float the velocity (BC must remain constant). Now calculate. The new chart will list your drop as exactly correct for 1000 yards. Now compare your info for the mid ranges with the new chart values. If the 500 yard value is higher or lower than your data value , bump the program BC up or down a few points and recalculate until your data matches . The BC now matches your bullet and the drop chart can be printed and used to make a custom turret. I don't know of any other way to get true BC and even though the chronograph and the mfg BC don't exactly match my chart, my bullets flight path does match. Also with this info I can change inputs for temp and altitude to produce charts for any hunt I'm going on.

 

[eddybo]

Yup, my point exactly, BCs do not matter. The only thing that matters is a number to plug into a calculator to match your actual drops. My only reason for continuing to bring it up is because of a fallacy some have, and that I once held. You cannot plug an accurate velocity into a calculator along with a Bc given by a manufacturer or otherwise and have things work perfectly. It is not that simple.

I have never shot the 200gr wildcats but would just about bet that that I would come closer with the .9 number than the actual BC if I plugged everything into my ballistic calculator. I come to that conclusion only because I know others that have used that number to make some really long range hits. I am not saying that the .9 is the true BC, but am saying it is the number that probably works best. The wildcats are a ULD bullet as are the bullet I have noticed a big difference between "working" and actual BC. I wonder if maybe there is a better form factor for ULDs just as there is a better form factor for VLDS.

What is the reason for knowing the exact BC ? When we use the BC and chronographed velocities to attempt to produce an accurate drop chart, we setting ourselves up for grief ! Manufactures BC's are nice and help us to get "on the paper" but don't address the variations of each gun. I just spent ten days at the eastern sports show trying to explain how to set up scopes for Best of the West and it seems that most shooters want an accurate drop chart but have bought the idea of the generic turret as being good enough. To get an extreamly accurate drop chart, several steps are necessary.
I use the program at the BOTW website with it's hugh list of bullets and BCs. Find your bullet and BC and begin entering your data. Scope height, click value, tempature, elevation, ect. Calculate to get a starting point and print out the chart to have as a range card.Now go shoot your data. I shoot in dead on at 200 and use the chart to find an on paper hold for 500 and 1000. The chart might suggest 64 clicks at 1000 and my gun might take 68 or 60. Write down your results. Do the same at 500 and as many other distances as you care to shoot. What we are doing is defining an arc for your bullet. Take these results back to the computer program and plug in the far target information and float the velocity (BC must remain constant). Now calculate. The new chart will list your drop as exactly correct for 1000 yards. Now compare your info for the mid ranges with the new chart values. If the 500 yard value is higher or lower than your data value , bump the program BC up or down a few points and recalculate until your data matches . The BC now matches your bullet and the drop chart can be printed and used to make a custom turret. I don't know of any other way to get true BC and even though the chronograph and the mfg BC don't exactly match my chart, my bullets flight path does match. Also with this info I can change inputs for temp and altitude to produce charts for any hunt I'm going on.

 

[Jon A]

What is the reason for knowing the exact BC ? When we use the BC and chronographed velocities to attempt to produce an accurate drop chart, we setting ourselves up for grief ! Manufactures BC's are nice and help us to get "on the paper" but don't address the variations of each gun. ….The chart might suggest 64 clicks at 1000 and my gun might take 68 or 60.

The problem with that is you are likely creating multiple "wrongs" to make a "right." Giving false inputs to get the data to match your observations. That's great if you plan to be hunting on the very same range you collected that data. In the same weather. With the same scope. That chart you made is all that matters.

But when you make the chart at sea level but are going to be hunting at 9000 ft, your chart is now worthless. The same false inputs will not give you the same correct outputs with the change in conditions--you can't just change the elevation in the ballistics program and expect to be correct. You need to shoot at every range and build another chart. Expecting the computer to correctly compensate for an atmospheric change using the wrong velocity, the wrong BC or the wrong click value just because you fudged those numbers to give correct results in a different atmosphere is expecting too much. Your click values won't change with the atmosphere, neither will your velocity but the computer is using those fudges in its baseline to figure the new dope.

I don't know of any other way to get true BC and even though the chronograph and the mfg BC don't exactly match my chart, my bullets flight path does match. Also with this info I can change inputs for temp and altitude to produce charts for any hunt I'm going on.

The way to do it is to make sure you have accurate inputs. Use a good chronograph. Use a bullet with a fairly certain G7 BC (like one BL has tested). Actually measure the click value of your scope.

If you were talking 1500 yds, a mile, etc, then yes, any little thing makes such a big difference it's really hard to be dead nuts on your first shot. But 1000 yds is not nearly so hard. If you put in accurate inputs the results will be accurate. If they aren't, figuring out the real reason your actual dope doesn't match the prediction is what you should try to do. Once you figure that out and everything matches up, you're in a much better position than you were with experimental fudged data. Now when you change the elevation in the program, it can figure new solutions using accurate data and will give accurate results. Or when the actual velocity does change in colder weather, you can actually enter the real velocity and get correct results. You can even switch scopes and enter the new, measured, click value and expect to be on the money.

 

[paphil]

Having shot identical rifles with identical amunition under identical conditions, I can tell you that one size BC does not fit all . I am not fudging inputs but using the exact data gathered from shooting and using the conditions at the time and place of the test. The first step of our process used the mfg BC and can include a chronograph . Very rarely will the test shot arc of flight fit the drop chart as predicted by the program. It will be close, for most close enough, but not exact. Some guys think hitting a 5 gallon bucket at 100 yards is good enough, I'm a little more particular. Once i've gone through all the steps and come up with a "true BC", The drop data will be exact out to bullet destabalazation at subsonic speeds and the program can use the data to calculate for additional altitudes and temperatures. A good groundhog gun that is dead on at 1000 yards but is 3 inches high at 500 is not very useful to me. It probably would be fine for big game. I am very happy with the results of the way we set up drop charts and if my chart matches my guns groups , why argue with success?

 

[Jon A]

Identical rifles shooting identical ammo with identical velocities, in identical conditions all having different trajectories points to scopes not tracking the values they advertise. If you think it's due to the bullets all having different BC's, you're fooling yourself.

 

[elkaholic]

Identical rifles shooting identical ammo with identical velocities, in identical conditions all having different trajectories points to scopes not tracking the values they advertise. If you think it's due to the bullets all having different BC's, you're fooling yourself.

I would have to agree with that being by far the most likely.......Rich

 

[ericpetritz]

Identical rifles shooting identical ammo with identical velocities, in identical conditions all having different trajectories points to scopes not tracking the values they advertise. If you think it's due to the bullets all having different BC's, you're fooling yourself.

Indeed. Why do some folks assume the BC must be incorrect if their predicted drops do not match actual drops? I realize some of the manufactures over state G1 BC's but for a long range style bullet using a Litz measured G7 BC? Isn't it more likely to be one of the other inputs? Such inaccurate scope tracking or inaccurate chronograph?

 

[paphil]

My main concern in setting up a drop chart is producing numbers from the data that exactly fit the flight of the bullet. That bullet comes with a specific shape that gives it a specific BC. To be useful to the gun operator, the bullet is exposed to a huge explosion and high pressure and is forced down a tight twisted tube, putting it under extreme stress. If that bullet shape changes in the slightest amount from those forces, so does the BC . I understand the new Berger 338 300 grain experienced such problems in development and had to have a stiffer jacket to prevent such deformation. I,m sure Brian could shed some light on that. Give me my gun and a turret for 5000 feet and using a few basic rules of thumb taught in our shooting schools and Iwill be able to shoot from 0 -10,000 feet and out to 1000 yards without using any calculator. It is all in getting set up correctly.

 

[ericpetritz]

My main concern in setting up a drop chart is producing numbers from the data that exactly fit the flight of the bullet. That bullet comes with a specific shape that gives it a specific BC. To be useful to the gun operator, the bullet is exposed to a huge explosion and high pressure and is forced down a tight twisted tube, putting it under extreme stress. If that bullet shape changes in the slightest amount from those forces, so does the BC . I understand the new Berger 338 300 grain experienced such problems in development and had to have a stiffer jacket to prevent such deformation. I,m sure Brian could shed some light on that. Give me my gun and a turret for 5000 feet and using a few basic rules of thumb taught in our shooting schools and Iwill be able to shoot from 0 -10,000 feet and out to 1000 yards without using any calculator. It is all in getting set up correctly.

Can you elaborate on your turret and rules of thumb?

 

[MontanaRifleman]

My main concern in setting up a drop chart is producing numbers from the data that exactly fit the flight of the bullet. That bullet comes with a specific shape that gives it a specific BC. To be useful to the gun operator, the bullet is exposed to a huge explosion and high pressure and is forced down a tight twisted tube, putting it under extreme stress. If that bullet shape changes in the slightest amount from those forces, so does the BC . I understand the new Berger 338 300 grain experienced such problems in development and had to have a stiffer jacket to prevent such deformation. I,m sure Brian could shed some light on that. Give me my gun and a turret for 5000 feet and using a few basic rules of thumb taught in our shooting schools and Iwill be able to shoot from 0 -10,000 feet and out to 1000 yards without using any calculator. It is all in getting set up correctly.

That change in shape is very insignificant in the vast majority of cases. Bullets are manufactured so identical that any possible difference in BC would be un-noticable. BC testing is done by firing and the results are from bullets that have been through the bore and any difference from one bore to the other is insignificant.

Bryan Litz is a professional in this field and probably one of top professionals in this field. He has education, training and experience in this field far beyond any of us. He has done more bullet testing than any ten (or 20... or 30... or...) of us put together. Why some folks get different BC's, I can't say for sure, but as has already been mentioned it's likely because of deviations with their equipement.... scope, chrony, etc.

Bottom line... accurately tested BC's are a very useful tool for the LR shooter. If in your testing, another number works for you then use it. Personally, I would go to great lengths to measure the accuracy of my equipement. I have two chrony's that differ by 5-60 fps (on the same string of shots) with no consistancy in difference. And when light conditons change, their readings change.

Bullet BC's do not change from bullet to bullet, lot to lot, or bore to bore.

-Mark

 

[paphil]

The turret is the same as shown on The Best Of The West and is set up in yards and includes wind deflection values. If I have a turret set up for 5000 feet , it will also have a stated temperature value along with bullet weight, BC and veolocity. For changes in altitude, at 1000 yards , allow 1/3 minute for each 1000 foot change . That is 1 click on the huskamaw scope. At 750 yards allow 1/2 click. For temperature it is the same allowance for each 20 degrees change , 1 click at 1000 yards and 1/2 at 750. For example my turret is set for 5000 feet elevation and 60 degrees F. I go hunting at 10000 feet at 0 degrees. for a 1000 yard shot, I would begin by dialing 1000 yards . Then dial down 5 clicks to compensate for altitude. next dial up 3 clicks to adjust the temperature. Aim and shoot. Run the numbers , it works. At 500 yards and less, make no adjustments. A turret made for 10000 feet would be more accurate but the temperature must still be a concideration,and adjusted for.