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.