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Variations in Ballistic Coefficients?

 
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ratassassin
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PostPosted: Fri Jan 02, 2009 11:36 pm    Post subject: Variations in Ballistic Coefficients? Reply with quote

Hi, guys. I've been running Al Bal to calculate trajectory maps for my .177 R7 and noticed that there are a variety of different possible BC's for the JSB Exact .177 8.4 gr. pellet on the web. Go to www.straightshooters.com and check the Velocity Ratings for various air rifles using the same pellet, and you'll find different BC's. For example, for the JSB Exact .177 8.4 gr pellet, I've found BC's listed of .014, .019, .020 and .021 depending upon the rifle. Similarly, the BC values reported by Straightshooters for the .20 JSB varies depending upon the rifle tested.

Obviously, what's changing from rifle to rifle is the muzzle velocity of the pellet during the tests. I believe BC is calculated using the initial muzzle velocity and a down range velocity in fps to determine average energy to come up with the BC. So this would mean that a pellet type will have different BC's depending upon the rifle shooting it. Now, I hasten to add that I could be completely wrong because I know almost nothing about ballistics. But I think that's what's accounting for the variability in BC reporting on the Straightshooters website, and why the trajectory programs are not giving real world results.

This makes some sense because the trajectory map I made for my R7 a few days ago did not mirror my experience in the field after 40 yards. I think I was using too low of a ballistic coefficient in the calculation and later found that by changing the BC in the program, I got dramatically different results that really start to matter as you go farther down range.

I remember reading that Robert Hamilton had reported that Chairgun wasn't giving him accurate trajectories to mirror real world experience until he increased the BC in the calculation, which probably resulted in the BC getting closer to how his gun was actually performing. Hence, the program started giving him more realistic numbers for POI.

Does this sound right? Sure would like your comments on this.

RA
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ratassassin
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PostPosted: Fri Jan 02, 2009 11:54 pm    Post subject: Reply with quote

My assumptions above may have been completely wrong. Wikipedia says the following about BC:

"In ballistics the ballistic coefficient (BC) of a body is a measure of its ability to overcome air resistance in flight. It is inversely proportional to the deceleration—a high number indicates a low deceleration. BC is a function of mass, diameter, and drag coefficient. It is given by the mass of the object divided by the diameter squared that it presents to the airflow divided by a dimensionless constant i that relates to the aerodynamics of its shape. Ballistic coefficient has units of lb/in˛ or kg/m˛. Normally BC's are stated in lb/in˛ by gun projectiles producers without referring to this unit."

So are the different BC numbers reported for the same pellet type due to differences from pellet to pellet in terms of their respective mass, diameter and drag coefficient?

RA
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Ca_Varminter
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PostPosted: Sat Jan 03, 2009 12:03 am    Post subject: Reply with quote

There are many different variables that affect BC. From pellet design to density. From velocity to choke/barrel design.
There also several ways of calculating actual BC. From initial to final Velocity. To even by the amount of Drop at a given distance. Do a Google search on it!!

Stuart
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ratassassin
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PostPosted: Sat Jan 03, 2009 12:32 am    Post subject: Reply with quote

Thanks, I have Googled it. But it's not very instructive as a practical matter. It seems to boil down to this. From what I've seen so far, it appears that BC can significantly vary from pellet to pellet within the same pellet brand and type, e.g., JSB Exact 8.4 gr. .177. Which, in turn, is causing me to conclude that programs such as Al Bal and Chairgun2 will be unable to accurately predict real life pellet trajectories for springers, particularly out past 40 yards.

I guess that's really the fundamental issue I was hoping to get feedback on.

RA
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Scott7161
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PostPosted: Sun Jan 18, 2009 2:24 am    Post subject: Ballistic Coefficient Reply with quote

I have more experience with this in the high power rifle world, but I've never heard of a ballistic coefficient changing for different velocities. The BC is for the specific bullet/pellet and the ballistics table/map should take into account the muzzle velocity so that you can plug your pellet in (and you know the MV since you measure it with a chronograph) and use the BC to calculate the pellet drop/drift in a given wind.

The BC should not change, it is a constant - that is my understanding at least.
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Ca_Varminter
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PostPosted: Sun Jan 18, 2009 3:11 am    Post subject: Re: Ballistic Coefficient Reply with quote

Scott7161 wrote:

The BC should not change, it is a constant - that is my understanding at least.


Well BC will vary according to temp, humidity and barometric pressure..etc.
But also it Will vary for a specific Velocity!!!
I am also a longtime HV Varmint hunter using anything from .20 tact to .22-250 AI both shooting in the +4000fps range.
If not certain about this check Sierra Bullet's site!! The should list a BC for a range of Velocities for a given bullet!! check varmint or match(MatchKing) bullets especially!

ATB,
Stuart
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Scott7161
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PostPosted: Mon Jan 19, 2009 5:28 pm    Post subject: Reply with quote

Quote:
Well BC will vary according to temp, humidity and barometric pressure..etc.
But also it Will vary for a specific Velocity!!!


Okay, I see that in the Sierra manual. I'm wondering how much of a difference velocity makes on BC for pellets.

Do you know if there is a good resource (similar to the Sierra Manual for bullets) that can be used for pellets?

Thanks!
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Ca_Varminter
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PostPosted: Mon Jan 19, 2009 6:26 pm    Post subject: Reply with quote

Well for the the Lowly Pellet....NO!!
Cry Cry Cry
The only thing you can do is compile your own chart!!
Using a programs like chairgun, that gives you External Ballistics results!

These programs derives a BC value by ether POI change between two known Distances. Or by Velocity differences at two Known Points!!

Stuart
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Scott7161
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PostPosted: Mon Jan 19, 2009 9:06 pm    Post subject: Reply with quote

Quote:
Well for the the Lowly Pellet....NO!!


Darn, okay well that will give me something to do! I'd take any recommendations about pellets that you think have a respectable BC though. I'm using Kodiak match pellets now. I started weighing them with my RCBS Rangemaster 1500 scale and I'm seeing a fair amount of variation in pellet weights from pellet to pellet. Makes me wonder if that might be affecting my consistency in point of impact.

Thoughts about a good pellet?
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Bayport_Bob
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PostPosted: Mon Jan 19, 2009 9:38 pm    Post subject: Reply with quote

A couple of days ago I measured the BC's of the 3 pellets that have been shooting well from my Air Arms S410 Xtra.

The setup was a Chrony Beta Master where I did an initial 5 string velocity measure at the muzzle and then moved the chrony to 25 yards for the 2nd velocity measurement with an additional 5 shots. I don't have room to spread the chrony anymore distance, so these are the best I can do with my setup...

Air Arms S410 .22 Full Power Setting

JSB Exact Express 14.3 Grain:

Distance 25 yards
Near Ave Vel. = 965.83
Far Ave. Vel. = 848.76
Difference = 117.07
BRC ED Value = .0242
Chairgun BC = .024
Muzzle Energy = 29.64 FtLbf
Energy @ 25 yds = 22.89 FtLbf

JSB Exact Jumbo 15.9 Grain:

Distance 25 yards
Near Ave Vel. = 931.62
Far Ave. Vel. = 824.67
Difference = 106.95
BRC ED Value = .0256
Chairgun BC = .026
Muzzle Energy = 30.68 FtLbf
Energy @ 25 yds = 24.04 FtLbf

JSB Exact Jumbo Heavy 18.1 Grain

Distance 25 yards
Near Ave Vel. = 879.6
Far Ave. Vel. = 799.9
Difference = 79.7
BRC ED Value = .0328
Chairgun BC = .033
Muzzle Energy = 31.13 FtLbf
Energy @ 25 yds = 25.73 FtLbf
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