Ballistic Coefficients and Accuracy?

[jm_cal]

Hi,

I am new to PCP rifles and new to the forum as well. If this topic was covered earlier or if it is a completely newbie question, I apologize. 

Goal is to find the best pellet for a gun. While that sounds simple and the answer maybe to try out as many as you can, I think the narrowing down process can be made more methodical if some existing data is used. Problem is I don't know yet, how to apply them and hence this thread. I got started with a RAW HM1000x .30 caliber. I have tried the JSB Exact .30 cal, 44,75 grain that the rifle is designed for and also the JSB .30 cal, 50 grain. Both of them performed really well at 100 yards with approx 1.5" groups. Wind being the major factor, I am wondering about the effect of ballistic coefficients on accuracy. 

To narrow down the search it would make sense to go with something that has the highest BC. Not sure if URLs are allowed in the forum, so google for "Ballistic Coefficient JSB" and look into the first link. 

The 0.25 cal JSB Exact King Heavy MK II 33 gr 

The 0.3 cal JSB exact Diablo 44.7 gr

and

The 0.3 cal JSB exact Diablo 50.15



immediately stand out with the 0.3 cal 50 grain coming out the highest at 0.053 BC. So if someone has a 30 cal rifle and if shot count or cost is not an issue and if only accuracy is, wouldn't they choose the 0.053 and just tune the gun to shoot it best?

Secondly, if you look at the velocity of the JSB exact 44.7 gr in the link "Hard Air Magazine", why is the 30 yard velocity at 782 fps which is much more higher than the 0.25 cal JSB Exact King Heavy MK II 33 gr, at only 603 fps at 30 yards?

It was all shot out of the same gun and I see an outlier here. Why am I asking this question?

The BC equation suggests, the higher the velocity, the better BC due to the velocity in the numerator. However there are other factors like harmonics. Let's just start with the first two questions and then proceed form there. 

Thanks,

Jai

 

[Macros]

BC and accuracy unfortunately have little to do with one another except in the scenario where your gun shoots two pellets identically as well as each other and one of them happens to have a significantly higher bc.

In that case, as you mentioned, long range accuracy should be easier to achieve with the higher BC projectile due to lower susceptibility to wind etc.

Even then though it's not always so simple as some pellets can destabilize at longer ranges compared to others etc.

It really can be like looking for the Holy Grail... And then sometimes it all lines up just right. Considering yours seems to work equally at 100 with both pellets you mentioned, maybe a bit of fine tuning for the 50's will do the magic. Only way to know is to try I suppose.



As far as the BC equation goes, I think you may be referring to the one for bullet shaped projectiles? Several guys here have mentioned that beyond a certain velocity pellet BC can actually drop due to the forces on the diabolo shape.

 

[functor]

In theory a higher BC projectile has a lower lag time( actual time to target- time to target in vacuum) and hence a lower wind deflection which is proportional to the lag time. 

So you end up having to face a theoretical quandary-- you could imagine a pellet with higher precision( small group size) and larger wind deflection ( low BC) versus another with less precision (larger group size) and smaller wind deflection ( high BC). Which would you choose? Obviously that is something up to the user and his ability to read wind. 

I personally would forget theory-- take the top 4-5 pellets, in practice almost always JSB and only sometimes H&N, and try to a few head sizes and shoot groups-- lots of them under identical conditions and look at the data and find the pellet with the smallest group size average and just stick to that. 

Unfortunately theoretical BC numbers don't always help in practice-- your particular barrel might shoot the high BC pellet horribly because of internal ballistics factors and harmonics. 

 

[jm_cal]

BC and accuracy unfortunately have little to do with one another except in the scenario where your gun shoots two pellets identically as well as each other and one of them happens to have a significantly higher bc.

In that case, as you mentioned, long range accuracy should be easier to achieve with the higher BC projectile due to lower susceptibility to wind etc.

Even then though it's not always so simple as some pellets can destabilize at longer ranges compared to others etc.

It really can be like looking for the Holy Grail... And then sometimes it all lines up just right. Considering yours seems to work equally at 100 with both pellets you mentioned, maybe a bit of fine tuning for the 50's will do the magic. Only way to know is to try I suppose.



As far as the BC equation goes, I think you may be referring to the one for bullet shaped projectiles? Several guys here have mentioned that beyond a certain velocity pellet BC can actually drop due to the forces on the diabolo shape.

Thanks for the insight.Yes, I am referring to the bullet shaped projectiles. Looks like there is a sweet spot for the velocity. I understand that accuracy is a factor of several other components. Assuming, that all other factors are held constant (shooter, gun, etc), wouldn't BC still be one of the critical components as wind is the biggest unknown?

 

[jm_cal]

Any thoughts on my second question regarding JSB 30 cal 44 gr velocity measurements posted?

 

[jm_cal]

In theory a higher BC projectile has a lower lag time( actual time to target- time to target in vacuum) and hence a lower wind deflection which is proportional to the lag time. 

So you end up having to face a theoretical quandary-- you could imagine a pellet with higher precision( small group size) and larger wind deflection ( low BC) versus another with less precision (larger group size) and smaller wind deflection ( high BC). Which would you choose? Obviously that is something up to the user and his ability to read wind. 

I personally would forget theory-- take the top 4-5 pellets, in practice almost always JSB and only sometimes H&N, and try to a few head sizes and shoot groups-- lots of them under identical conditions and look at the data and find the pellet with the smallest group size average and just stick to that. 

Unfortunately theoretical BC numbers don't always help in practice-- your particular barrel might shoot the high BC pellet horribly because of internal ballistics factors and harmonics.

The last point you mention is what makes this challenging and also fun  

 

[Macros]

BC and accuracy unfortunately have little to do with one another except in the scenario where your gun shoots two pellets identically as well as each other and one of them happens to have a significantly higher bc.

In that case, as you mentioned, long range accuracy should be easier to achieve with the higher BC projectile due to lower susceptibility to wind etc.

Even then though it's not always so simple as some pellets can destabilize at longer ranges compared to others etc.

It really can be like looking for the Holy Grail... And then sometimes it all lines up just right. Considering yours seems to work equally at 100 with both pellets you mentioned, maybe a bit of fine tuning for the 50's will do the magic. Only way to know is to try I suppose.



As far as the BC equation goes, I think you may be referring to the one for bullet shaped projectiles? Several guys here have mentioned that beyond a certain velocity pellet BC can actually drop due to the forces on the diabolo shape.

Thanks for the insight.Yes, I am referring to the bullet shaped projectiles. Looks like there is a sweet spot for the velocity. I understand that accuracy is a factor of several other components. Assuming, that all other factors are held constant (shooter, gun, etc), wouldn't BC still be one of the critical components as wind is the biggest unknown?

BC is definitely a critical part of the whole thing, just unfortunately so tangled up with the other variables that it can get very difficult.

Personally at airgun ranges I prefer to go with the projectile that offers the most precision in still conditions, regardless of bc (although I'm of course happier when it's high). The reason for this is to my mind, wind is at least somewhat measurable and one can hone their skills in dealing with it. Conversely no amount of practice is going to help with the effectively random scattering of my shots with a less precise pellet/slug. Plus on a still day in particular, those bigger groups and near misses would irritate the living daylights out of me.

There are definitely people out there that feel differently than me (on many things, lol). I'll be interested to see what ends up working for you!

 

[jm_cal]

BC and accuracy unfortunately have little to do with one another except in the scenario where your gun shoots two pellets identically as well as each other and one of them happens to have a significantly higher bc.

In that case, as you mentioned, long range accuracy should be easier to achieve with the higher BC projectile due to lower susceptibility to wind etc.

Even then though it's not always so simple as some pellets can destabilize at longer ranges compared to others etc.

It really can be like looking for the Holy Grail... And then sometimes it all lines up just right. Considering yours seems to work equally at 100 with both pellets you mentioned, maybe a bit of fine tuning for the 50's will do the magic. Only way to know is to try I suppose.



As far as the BC equation goes, I think you may be referring to the one for bullet shaped projectiles? Several guys here have mentioned that beyond a certain velocity pellet BC can actually drop due to the forces on the diabolo shape.

Thanks for the insight.Yes, I am referring to the bullet shaped projectiles. Looks like there is a sweet spot for the velocity. I understand that accuracy is a factor of several other components. Assuming, that all other factors are held constant (shooter, gun, etc), wouldn't BC still be one of the critical components as wind is the biggest unknown?

BC is definitely a critical part of the whole thing, just unfortunately so tangled up with the other variables that it can get very difficult.

Personally at airgun ranges I prefer to go with the projectile that offers the most precision in still conditions, regardless of bc (although I'm of course happier when it's high). The reason for this is to my mind, wind is at least somewhat measurable and one can hone their skills in dealing with it. Conversely no amount of practice is going to help with the effectively random scattering of my shots with a less precise pellet/slug. Plus on a still day in particular, those bigger groups and near misses would irritate the living daylights out of me.

There are definitely people out there that feel differently than me (on many things, lol). I'll be interested to see what ends up working for you!

Will let u know . I always wonder what people mean when they say the gun is more accurate than you  

 

[Centercut]

Accuracy always trumps BC. If accuracy is equal then BC becomes the determining factor.