Let's argue about BC's

[ericpetritz]

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.

That is interesting. I too have similar BDC type turrets on some rifles but I always get anal about it when the conditions are different from where the turret is calibrated, especially at long range. This leads me to lean towards using rifles without BDC turrets. I will have to compare these rules of thumb versus actual drop predictions on my rifles and see how close it is. Relative to the current discussion, it is still most accurate to ensure the turret is calibrated with inputs that are as accurate as possible including BC's, velocity, environmental conditions, etc.--wouldn't you agree?

 

[eddybo]

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 conssitancy in difference. And when light conditons change, their reading change.

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

-Mark

I agree accurately measured BCs are great......but find that they are most accurately measured by the barrel you are going to be shooting that in.

I have on numerous occassions found one bullets BC to be very accurate, then when switching to another manufactuerers bullets in the same rifle found the BC to be very inconsistant with the trajectory of the rifle. A situation such as that, especially in a situation where ammo with both bullets are being shot through the same chronograph within a span of only a few hours is disheartening to one's beleif that BCs are infallable.
If you want to go past 1K or heck even to 800 BCs are not plug and play, the gun has to be shot.

 

[elkaholic]

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.

That all makes perfect sense as long as the barometer doesn't change. Altitude is nothing more than barometric pressure so you MUST have a pressure reading to be consistent....Rich

 

[rscott5028]

I salute Paul and all of the experts who've contributed to this thread from which I've learned a lot.

Unfortunately, everything to do with modern rifles, scopes, and ammunition is about tolerances. ..not to mention my own inability to accurately measure and judge atmospheric conditions.

As such, I'm often amazed at just how predictably I can shoot out to 1k. And, it's mind boggling to think that some of you guys shoot more than twice that far.

While it's not imperative that I know the perfect BC to get dialed in, it is hugely beneficial to the long term improvement of shooting sports that people continue to split hairs and think critically.

Perhaps some of us will live to see the day that one can purchase a rifle off the shelf and attach the optic of choice and shoot the X ring at 1k without having to bore sight or collect dope on the rifle?

Until then, realistic mfg G1 and/or G7 BCs are the best tool we have prior to squeezing the trigger. Those makers who consistently publish unrealistic BCs do so at their own peril.

thanks!
richard

 

[LR3]

Grouper. Thanks for your comments. I ve read Bryan Litz,s book but I missed seeing his G 7 or 1 values if they were published for 350 gr smk. I started with the bc values Kirby suggested for the 375 AM of .77 or so and ended up using .835 and that tracks well out to 1 mile for all distances for the 350gr. For the 375 sniptac I found changing brass and load changed the bc from8 to .85bc. So I agree velocity is a big factor for G1. I also found that like you my 5.5-22 nfs scope increased moa more than marked. The Sand B tracked better on testing at 100yds. Clearly accurate scope input is critical. The AM is accurate for me to 9 inches at a mile but Kirby did better. The snipetac is nearly as good but has a lower velocity, is lighter, and has a shorter barrel and is more a carry hunting rifle. Kevin, yes the bullets are from defferent batches so here may differences. I measure temp, pressure, humidity etc before setup with Kesterel and Exbal input. For the Snipetac the bc has held true for 1000ft out to a mile and to 10,000 ft (for a 636 yd elk two shots mid chest 4 inches apart, and 1000yd rock popping). The point is I think that every bc has to be calculated to match rifle and load and bullet and scope beyond 1000 yds if using G1 values since velocity and scope inaccuracies are potential issues as Grouper pointed out. I hope Bryan will add his comments.

 

[Michael Eichele]

What I am about to post in both words and info chart, if I am wrong I would like somebody to set me straight. Tell me, better yet, show me what I am doing wrong.

1st: I believe BC's change with velocity be it G1 or G7 or any other drag model you choose to use.

2nd: I believe that BC's change from rifle to rifle (albiet it is not a huge change).

3rd: I do not believe that Litz's BC's are fool proof.

4th: Rarely do I ever get a published BC to work 100% correctly. Or BL's for that matter. I usually find that BL's and/or published BC's are within 3-8% compared to what I determine for my rifle. On a few occasions they are more. The worst I have had was 17.5% away from published.


1: Why do I believe that BC's change with velocity be it G1 or G7? It is a long story. I will post a chart below for clarification. In other words, I will get to that later.

2:Why do I believe that BC's change from rifle to rifle? I will say that after working up loads for a given rifle and testing them from every angle available to me and being able to consistently predict without fail in fact, a bullet drop on any given day out to 1K. This works for the life of the rifle with the exception of very very minor fine tuning and adjustments over the course of it's usefull life. Then, rebarrel the rifle. Same cartridge, similar twist etc......Work up a new load using the same bullets. Sometimes even the same lot. Get a few days in at the range fine tuning everything and shooting every shot over the same chrony as used before on the same range during the same time of year off the same bench in the same light. Using the same base, rings, scope, only changing the physical barrel. After all the data is collected, go to the longer ranges. Using the same BC's as previously used, only to find that it does not work. Close? Sure, sometimes. The same? No. I will use a slightly different BC for the LIFE of that barrel and still continue to make consistent first round hits with unfailing predictability. I will concede that sometimes the differences are extremely small. So much so that I could admit that it may just be minor inconsistencies induced by me. One bullet was within 1/2 MOA at a full 1K from barrel to barrel. Yet, with another bullet they was far enough apart that I would be scared as he!! to say for fear of recieving major flame here. Maybe the fact that this bullet was fired at 200 FPS difference between barrels (different powder and charge) had something to do with it. Maybe? Alright, I will take the heat. 5.5% difference. That is G7 for G7. Litz's and mine are 7% apart. Please, by all means, show me where I am going wrong here. What else changed?

3: I do not believe that Litz's figures are fool proof. Do I believe he does a great job? Absolutely. Do I feel he is infallable? No. Many times he has admitted himself that he has made mistakes and has corrected his stated values. He should be commended for admiting the wrongs. Again, this is not to pick on him or beat him up. It just illustrates that he makes mistakes too. Couple that with the fact that there are so many parameters that can affect BC's and they just are not 100% predictable from user to user, velocity to velocity. Published AND Litz's BC are a great place to start but neither are fool proof. That said, there has been one bullet that was within 3" at 1K between my tests and Litz's. Obviously we both did something right. The 190 SMK. This figure (.272 G7) was also scarey close between two barrels but not identical. Again, if I am wrong here, please tell me how. Better yet, show me.

4: Right or wrong, the fact is that I rarely if ever find a published BC or a Litz BC to match my results be it from double chronies and/or drop tests. That is a fact. Am I doing something wrong? I would like to think not but if I am I am going to have an open mind. Please, show me where I am going astray here. I would love to say that all of my BC's figured are exactly 3.5% or at least within a 2-4% zone off of Litz's or published BC's. Then I would know that something was consistently wrong with my methods. When there are variences from 3% to almost 18% and everything between (mostly 3-8%), I have a hard time believing that the entire problem is with me. Is there a problem with the way I come up with BC's? Probably. In fact, for certain. But when those variences are so wide, I will all but refuse to buy into the possibility that the entire problem is with me. Litzs may be good at what he does but give some of the guys on this site some credit too.

Back to #1: Below is a chart of actual tests from two different rifles. All of the same methods were used to 'fine tune' my drops. Same equipment, same location. Same bore diameter, different cartridge, different barrel lengths and twist and of course different velocities all done within a few days of eachother, even using the same scope. The differences is BC is quite noticable. For the rest of the life of each barrel, their drops were 100% predictable and reliable out to 1K. Actually, the 300RUM was only ever shot out to 850. The 308 was shot to a full 1000.

The chart on the upper left is the actual test data for the 308. The tests were done over multiple days. The last confirmation day is the conditions recorded on this chart. On the upper right, the same holds true for the 300RUM. Then below each respectively is the drop data from the JBM website using Litzs BC's. Both are using a G7 BC. Both are very different. The only real differences here are velocity, barrel twist which according to what I have been reading in this thread are absolutely meaningless. Please for the love of goodness show me what I am doing wrong.

For the record, this is NOT to try and show that anybody posting here is wrong. Nor is it an argument in any way. The results above and below are no joke. The result whether or not they were obtained properly are factual. I am honestly trying to show my real world results and wondering if any of you can teach me something that I MAY be missing.

 

[eddybo]

Micheal,

I for one do not think you are doing a thing wrong, other than doing enough shooting to realize that given BCs are just a starting point and that they are not magically taylored to your specific rig.

 

[ericpetritz]

What I am about to post in both words and info chart, if I am wrong I would like somebody to set me straight. Tell me, better yet, show me what I am doing wrong.

1st: I believe BC's change with velocity be it G1 or G7 or any other drag model you choose to use.

2nd: I believe that BC's change from rifle to rifle (albiet it is not a huge change).

3rd: I do not believe that Litz's BC's are fool proof.

4th: Rarely do I ever get a published BC to work 100% correctly. Or BL's for that matter. I usually find that BL's and/or published BC's are within 3-8% compared to what I determine for my rifle. On a few occasions they are more. The worst I have had was 17.5% away from published.


1: Why do I believe that BC's change with velocity be it G1 or G7? It is a long story. I will post a chart below for clarification. In other words, I will get to that later.

2:Why do I believe that BC's change from rifle to rifle? I will say that after working up loads for a given rifle and testing them from every angle available to me and being able to consistently predict without fail in fact, a bullet drop on any given day out to 1K. This works for the life of the rifle with the exception of very very minor fine tuning and adjustments over the course of it's usefull life. Then, rebarrel the rifle. Same cartridge, similar twist etc......Work up a new load using the same bullets. Sometimes even the same lot. Get a few days in at the range fine tuning everything and shooting every shot over the same chrony as used before on the same range during the same time of year off the same bench in the same light. Using the same base, rings, scope, only changing the physical barrel. After all the data is collected, go to the longer ranges. Using the same BC's as previously used, only to find that it does not work. Close? Sure, sometimes. The same? No. I will use a slightly different BC for the LIFE of that barrel and still continue to make consistent first round hits with unfailing predictability. I will concede that sometimes the differences are extremely small. So much so that I could admit that it may just be minor inconsistencies induced by me. One bullet was within 1/2 MOA at a full 1K from barrel to barrel. Yet, with another bullet they was far enough apart that I would be scared as he!! to say for fear of recieving major flame here. Maybe the fact that this bullet was fired at 200 FPS difference between barrels (different powder and charge) had something to do with it. Maybe? Alright, I will take the heat. 5.5% difference. That is G7 for G7. Litz's and mine are 7% apart. Please, by all means, show me where I am going wrong here. What else changed?

3: I do not believe that Litz's figures are fool proof. Do I believe he does a great job? Absolutely. Do I feel he is infallable? No. Many times he has admitted himself that he has made mistakes and has corrected his stated values. He should be commended for admiting the wrongs. Again, this is not to pick on him or beat him up. It just illustrates that he makes mistakes too. Couple that with the fact that there are so many parameters that can affect BC's and they just are not 100% predictable from user to user, velocity to velocity. Published AND Litz's BC are a great place to start but neither are fool proof. That said, there has been one bullet that was within 3" at 1K between my tests and Litz's. Obviously we both did something right. The 190 SMK. This figure (.272 G7) was also scarey close between two barrels but not identical. Again, if I am wrong here, please tell me how. Better yet, show me.

4: Right or wrong, the fact is that I rarely if ever find a published BC or a Litz BC to match my results be it from double chronies and/or drop tests. That is a fact. Am I doing something wrong? I would like to think not but if I am I am going to have an open mind. Please, show me where I am going astray here. I would love to say that all of my BC's figured are exactly 3.5% or at least within a 2-4% zone off of Litz's or published BC's. Then I would know that something was consistently wrong with my methods. When there are variences from 3% to almost 18% and everything between (mostly 3-8%), I have a hard time believing that the entire problem is with me. Is there a problem with the way I come up with BC's? Probably. In fact, for certain. But when those variences are so wide, I will all but refuse to buy into the possibility that the entire problem is with me. Litzs may be good at what he does but give some of the guys on this site some credit too.

Back to #1: Below is a chart of actual tests from two different rifles. All of the same methods were used to 'fine tune' my drops. Same equipment, same location. Same bore diameter, different cartridge, different barrel lengths and twist and of course different velocities all done within a few days of eachother, even using the same scope. The differences is BC is quite noticable. For the rest of the life of each barrel, their drops were 100% predictable and reliable out to 1K. Actually, the 300RUM was only ever shot out to 850. The 308 was shot to a full 1000.

The chart on the upper left is the actual test data for the 308. The tests were done over multiple days. The last confirmation day is the conditions recorded on this chart. On the upper right, the same holds true for the 300RUM. Then below each respectively is the drop data from the JBM website using Litzs BC's. Both are using a G7 BC. Both are very different. The only real differences here are velocity, barrel twist which according to what I have been reading in this thread are absolutely meaningless. Please for the love of goodness show me what I am doing wrong.

A few comments/questions:

I think you are correct, all BC's have some velocity dependence. This dependence is minimized the closer the bullet in question matches the standard to which you are using. For example, VLD type bullets that are the subject of this thread will have much less velocity dependence when using a G7 curve than they will when using a G1 curve because the G7 standard projectile very closely matches these types of bullets. Trying to use a single G1 BC with a typical long range style bullet will result in a great deal of velocity dependence. This is why Sierra uses multiple velocity banded G1 BC's. There is no doubt that some of the manufacturers post inflated G1 BC's that would only be accurate at very high velocity.

Why do you believe that the BC's are the result of the differences/errors in your drop charts? Isn't it just as likely that your chronograph is displaying inaccurate velocities? Most all of the inexpensive chronos on the market will have a significant accuracy tolerance. Have you calibrated the scopes on both rifles to ensure they track accurately? Even on my Nightforce scopes I've found errors in how accurately the scopes track. These errors are not significant at close range but are significant way out there. How close are your field zeroes relative to the charts, in other words are you "right on" at your intended short range zero?

I get the feeling that many folks automatically assume the BC is the problem when their actual drops do not match the predicted drops from the ballistic software. Why is that? Is it because manufacturers have historically over inflated these numbers? Is it because we use chronos and it gives us the feeling of a "dead on" velocity measurement? Is it because BC is the least intuitive of the inputs so we feel like it must be the BC? It is intriguing to me. No doubt Mr Litz is fallible as we all are, but his methods of measuring BC's are scientifically sound and subject to a low error if done correctly.

I am not saying you are dong anything wrong, just trying to understand what is happening. This is a great thread and discussion!

 

[Michael Eichele]

A few comments/questions:

I think you are correct, all BC's have some velocity dependence. This dependence is minimized the closer the bullet in question matches the standard to which you are using. For example, VLD type bullets that are the subject of this thread will have much less velocity dependence when using a G7 curve than they will when using a G1 curve because the G7 standard projectile very closely matches these types of bullets. Trying to use a single G1 BC with a typical long range style bullet will result in a great deal of velocity dependence. This is why Sierra uses multiple velocity banded G1 BC's. There is no doubt that some of the manufacturers post inflated G1 BC's that would only be accurate at very high velocity.

Why do you believe that the BC's are the result of the differences/errors in your drop charts? Isn't it just as likely that your chronograph is displaying inaccurate velocities? Most all of the inexpensive chronos on the market will have a significant accuracy tolerance.

See answer below

Have you calibrated the scopes on both rifles to ensure they track accurately? Even on my Nightforce scopes I've found errors in how accurately the scopes track. These errors are not significant at close range but are significant way out there. How close are your field zeroes relative to the charts, in other words are you "right on" at your intended short range zero?

See answer below

I get the feeling that many folks automatically assume the BC is the problem when their actual drops do not match the predicted drops from the ballistic software. Why is that? Is it because manufacturers have historically over inflated these numbers? Is it because we use chronos and it gives us the feeling of a "dead on" velocity measurement? Is it because BC is the least intuitive of the inputs so we feel like it must be the BC? It is intriguing to me. No doubt Mr Litz is fallible as we all are, but his methods of measuring BC's are scientifically sound and subject to a low error if done correctly.

I am not saying you are dong anything wrong, just trying to understand what is happening. This is a great thread and discussion!

The questions I have lay with the fact that I have been using the same physical chronies for several years. We have alot of cloudy days here. I do all of my serious and documented testing with cloud cover for consistency.

The reason so many inconsistencies baffle me is due to the fact that I do the same thing using the same chronies and often times the same scope transfered from one rifle to the next in similar lighting for each rifle and load. In some cases, using the same rifle/scope/bullet with a new barrel. I will say that when the velocity and twist is similar, the BC is similar from barrel to barrel. When there is a major barrel and/or velocity difference, I see much bigger changes in the BC. I say BC because changing only the barrel and velocity changes my predicted drops so that the BC value is the only variable that has to change to make my drops work. Aside from using a new velocity in the calculator.

Calibrated, Yes. In addition, in many tests, even between different rifles, the same physical scope was used. Same rings, using picatinny rails on each rifle and re-sighted.

 

[eddybo]

While the wildcat are secant ogive they have a rebated boat tail and are termed as uld.

 

[paphil]

I have to agree with you Mike, once you have your data and can make it work with a program to give good results, why spend time worrying about it ? I want a drop chart that fits my bullet flight path , Might mention also that the short range zero is critical and is easier to be off a click and mess up everything. This is a great thread and hopefully makes people think.

 

[groper]

It seems some of you still arnt getting it...

Michale echel, eddy, and others... the reason your seeing the discrepancies from rifle to rifle and bullet to bullet, and have to keep changing the BC to get your trajectory right - IS PRECISELY BECAUSE your not starting with the correct BC in the first place!!! You really need to let that sink in...

Additionally, manufacturer quoted BCs are only there to sell bullets and should be ignored... they artificially inflate them, and are almost always quoted as a "typical muzzle velocity G1 number "- totally useless for developing a trajectory from as the bullet slows down the second it leaves the muzzle, and the G1 BC goes south with it...

Once you get the BC right, everything falls into place like magic, no matter which rifle you shoot that bullet from, where you shoot it, or who pulls the trigger. Getting it right is the extremely hard part and just because you got your trajectory to line up at your range/backyard, doesnt mean youve found an accurate BC for that bullet.

You really need to be a proper ballistician with proper equipment to derive the average BC accurately - which is why i keep bringing up bryan litz, thats what he does and does it well.

 

[elkaholic]

I've been sitting back reading everything and have learned a few things but have also come to some conclusions. First of all, I have heard of several good methods to determine B.C. . I have also heard of some very good ways which I feel are ways to make "SUBTLE" changes to an accurate starting B.C. Having said that, I have to agree with Nick "groper" that an accurate B.C. to begin with is critical. If for no other reason, some of your comments have proven just that without intending to. By that I mean, if you find some major deviation in your drop charts, you know that you have an input error of some sort! It seems to me that if we don't do this, we are reinventing b.c. every time we change something. I think a lot of this problem has come about because manufacturers have been deceitful, at best, as to accurate b.c. Instead of political spin, its bullet spin. Once the lack of trust is there by we shooters, and rightfully so, we do ALL kinds of things to make the necessary corrections (me included). What we should be doing is using an unbiased pros opinion, "you can put whoevers name you want" and start with that. If we don't do that, why even mention b.c.? Just go out and shoot your rifle at every distance and you can and adjust your scope turrets, drop charts or whatever accordingly. Nothing wrong with that!
I don't mean to be disagreeable or arguementive because some VERY good info came out in this thread by some guys more knowledgeable than myself. I just think if we are going to use b.c., we should start with an accurate one. I also think we would be better off doing away with G1 for any of the long range hunting applications. My opinion....Rich

 

[Michael Eichele]

It seems some of you still arnt getting it...

Michale echel, eddy, and others... the reason your seeing the discrepancies from rifle to rifle and bullet to bullet, and have to keep changing the BC to get your trajectory right - IS PRECISELY BECAUSE your not starting with the correct BC in the first place!!! You really need to let that sink in...

Additionally, manufacturer quoted BCs are only there to sell bullets and should be ignored... they artificially inflate them, and are almost always quoted as a "typical muzzle velocity G1 number "- totally useless for developing a trajectory from as the bullet slows down the second it leaves the muzzle, and the G1 BC goes south with it...

Once you get the BC right, everything falls into place like magic, no matter which rifle you shoot that bullet from, where you shoot it, or who pulls the trigger. Getting it right is the extremely hard part and just because you got your trajectory to line up at your range/backyard, doesnt mean youve found an accurate BC for that bullet.

You really need to be a proper ballistician with proper equipment to derive the average BC accurately - which is why i keep bringing up bryan litz, thats what he does and does it well.

What you dont seem to get is when some of use the Litz BCs or any other BC for that matter we hit nothing but air. And a lot of air sometimes.

You are quick to tell me that I am not using the right BC in the first place but you do nothing to tell me how.

This is what I am asking you.......Show me how to get the THE RIGHT BC IN THE FIRST PLACE!!!!

Also, I am NOT talking about G1 BC's here. I am refering to G7's. They still change with velocity. Please show me what I am doing wrong. Dont tell me I am doing it all wrong if you cannot or will not show me how to do it right. Remember, I am not arguing with you or anybody else. Just asking that you show me what I am doing wrong. Show me why when I use Litz's BC's for 1 bullet its nearly spot on and for most others it is not close.

PS, please dont butcher my name. I will offer you the same courtesy.