Calculating BC from pellet drop

[SagaciousKJB]

Say I wanted to use ChairGun to calculate the BC of pellets using the POI drop.  Well, suppose I can't shoot much better than a 1/2" group at 18 yards, then at the "test range" ChairGun wants me to shoot at it will probably be an even looser group.  Question is, how do you use such a large group to properly input the drop?  Just measure the center of the group that you shot and use that?  Will that really give me an accurate figure for BC?

[rsterne]

You have to use the center of the group, so the accuracy of the calculation will depend on the group size and how accurately you determine the center.... Velocity downrange is a much better method, IMO, providing you don't shoot your Chrony....

Bob

[Doug Wall]

Is your pellet on here?

http://www.photosbykev.com/wordpress/userfiles/pelletdata.htm

[SagaciousKJB]

Is your pellet on here?

http://www.photosbykev.com/wordpress/userfiles/pelletdata.htm

I don't believe it's the same type.  The ones I have are Crosman Pointed Field Hunting pellets, but they're 7.4 grain.  And those mention something about Copperhead, but not these.

[MicErs]

Say I wanted to use ChairGun to calculate the BC of pellets using the POI drop.  Well, suppose I can't shoot much better than a 1/2" group at 18 yards, then at the "test range" ChairGun wants me to shoot at it will probably be an even looser group.  Question is, how do you use such a large group to properly input the drop?  Just measure the center of the group that you shot and use that?  Will that really give me an accurate figure for BC?

A long time ago (1992) I bought a couple of fine air rifles and wrote some software to compute BC by measuring drop over range and then experimenting with BC in the formula until I had something that matched the data I saw on the range.  That was when I started shooting Premiers because that pellet just flies well.  I probably shot five different pellets in .177 and .22 each.

Your question is one of statistics.  If you want really good information, you need to collect a lot of groups and analyze the data.  You will get more accurate information if you shoot at two widely spaced distances. 

[SagaciousKJB]

Well I shot about 20 shots at a target at 40 yards, but like I said I'm barely able to shoot very tight groups at the 18 yard zero range.

So anyway what I wound up with was three different groups...  One was oddly right on line with the bullseye elevation wise, but a little to the left.

The second was half an inch down, and the third 1 1/2" down.

Well I decided that the ones that were level with the bullseye couldn't be right, and the ones only half an inch down gave a BC that was way too high, and the group 1 1/2 down way too low.  So I split the difference and calculated it for an inch and it was .0209 which is more along the lines of the BC figures for the crosman pellets in the database so I think I'm going to use that for now--looking at the reticle simulator it's right on for where I need to aim at 30 yards and a 15 degree incline.