I know it sounds like i'm beating a dead horse here, but I have seen posts on "what is the BC of such and such pellet". How can you tell a BC by pellet alone? I thought you couldn't do that and that it was based on the gun & pellet?
You are correct Robert - it's a combination of the gun, pellet &
January 12 2006, 10:55 PM
current atmospheric conditions. However, after a while, folks get to talkin, and while some may do better & some may do worse, an average BC for a pellet gets estabilished. It's not clear what the average BC for the new JSB heavies is yet, but my rifle shoots them for a BC of 0.035 (estabilished w/ Chrony readings @ 10 & 30 yards). The pubilished BC or "estabilished" BC is a good # to compare BC's w/ against other pellets, but will have to be refined for your rifle. The BC of 0.035 for the JSBH's is clearly better than the BC for JSB mediums (0.022), meaning at longer ranges (all else being equal) the JSBH shoots flatter & is less susceptible to wind drift than the JSBM.
The avg. BC (like 0.022) JSBM's is usually close enough to work pretty well - difference of 0.001 to 0.002 in BC don't usually affect trajectory to 55 yards very much for practical purposes (less than 1/8" at 55 yards w/ JSBH's @ 940 fps according to Chairgun2) and are certainly close enough to do what everyone says you should do anyway: Check your ballistics program #'s at the range!
Different BCs at different distances from same gun/pellet?
January 13 2006, 8:36 AM
I did some bc testing once, and it seemed that depending on where the velocity readings were taken, the bc would change.
This was with cp lights at around 780 starting velocity. I don't have the numbers, but memory is that for example, first reading @ 2 yds and second @ 20 yds gave a different bc than first reading @ 10 yds and second @ 30 yds. Then, a third different result if first @ 2 yds and second @ 30 yds. I only have one chrony, and the way i did the test may have caused this. I shot 10 shots, and then moved the chrony to second position and shot 10 more shots. I used average of these 2 strings to compute bc. I know the longer a distance used is supposed to make bc calculation more accurate, but it makes me wonder if the pellet is shedding velocity at a non-linear rate and this is giving different bc numbers at various places along the way? If that's true, then it would seem that if a person was anal about it, he would need to compute his bc at each different yardage he was going to shoot, for each elevation and climatic condition he would encounter.
Now, I realize this all might not make one whit of difference in the real world of shooting but I found it curious and wondered if there was error in my method or if this actually occurs?
BC is basically a function of initial velocity and the shape of the pellet. The shape of the pellet is a function of the shape of the pellet when you load it into the breech, how much the 'firing cycle distorts the pellet, and any additional shaping contributed by the barrel.
You are right about the anal part, but most of it is not of much consequence in the real world.
If you start applying high powered math to the problem there are lots of other variables that contribute practically nothing.
BC typically changes some as you progress down range but many of those factors arent easily measured and arent to significant as applied to airguns. The average BCs we come up with(using down range chrony readings) address most of these issues.
Thanks guys, I figured I was doing something wrong.
January 13 2006, 12:42 PM
So, if you just use starting velocity and pick a distance for the second reading that's sufficiently far away, (how far is sufficient anyway?), and use careful measurement with a good BC calculating program, then everything will work out and BC for that gun/pellet will be accurate for all ranges then?
Also, thanks for the reminder about Harry's work - I remember it now that you bring it up and will search back to read it again. From what I recall it was incredibly close between prediction and real world results. If we had POMs back then that one would have surely won the month.
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