For those of you following my threads regarding ballistic coefficient testing (primarily on .25 cal and .30 cal pellets, through polygon (.25 cal) and LW rifled (.25 & .30 cal) barrels), some of you may have read that I've gone ahead and purchased a second chronograph to complete testing more efficiently, and perhaps more accurately (since the BC for a single pellet can be measured directly). So after a long wait for my new chrony to arrive (the Caldwell Ballistic Precision Premium Kit), I finally set off to begin calibrating the two chronographs to each other so I know how much to offset the readings from one to match the other, so I can accurately start using them both to test BC's as I intend. I've decided to share the data here along with a few of my observations to provide a service to the community, and because I see a few things in the data that I didn't anticipate-and lastly because I'm not totally convinced I have the correct 'offset' number between the two chronies to use moving forward (your assistance is welcome)...
Method:
To start off with, I decided to position the first chrony in my standard (1 yard) position, and the second one a set distance behind it (18", measured center of rear window to center of rear window). The first chrony was positioned so that the 'rear' sensor window measured 36" from the end of my moderator. The second unit measured an additional 18" behind the first (or 1.5 yards from the end of the moderator). From there, I emptied 1 full .25 caliber magazine (shooting JSB Heavy, Mk I pellets) across both chronographs, recording the data, shot by shot, from both of them (later downloading into a spreadsheet for comparison).
The first magazine full was shot with the Caldwell unit at 1 yard, and the Shooting Chrony unit at 1.5 yards. After that, I switched the sequence and placed the Shooting Chrony Beta unit in at 1 yard, and the Caldwell unit at 1.5 yards from the end of the moderator, and proceeded to shoot a magazine full of lead over both units, record, etc.
The Data:
Now for the interesting part-the data:
Observations:
So here are a few things that stand out to me:
1) The data shows that the pellets are not fully stabilized at 1 yard from the muzzle, as evidenced by both chronographs showing a much larger extreme spread and standard deviation when positions 1 yard from the muzzle, to 1.5 yards from the muzzle (16 or 17 fps @ 1 yard vs.. 10.7 or 10 fps @ 1.5 yards). This, to me is a very significant finding--and perhaps should provide some measure of comfort to many shooters-regardless of what gun, ammo, etc they are shooting-that their gun is shooting just fine. Too often I read about people complaining about higher standard deviations when checking out a new gun, yet the gun shoot excellent groups at 50 yards (being a RAW shooter, I've read this from RAW owners, but also from a bunch of FX owners, as well as many other brands). Perhaps, as the data here suggests, the variation really shouldn't be taken a too much of a concern until one is sufficiently far enough from the end of the gun to ensure the Extreme spread and STD have settled down to represent what the gun is actually doing (when the pellet is stabilized). Since I have just discovered this (for myself) today, I don't have the answer on the 'ideal' distance from the muzzle to locate the chronograph for readings that represent stabilized pellets (the barrel may have an impact on that distance as well as the pellet, perhaps?).
2) I'm not sure I've settled on the offset number to use between the two chronographs-but the Caldwell consistently does read higher than the Shooting Chrony Beta unit - by about 10 fps. From the data I've shown above, at the 1 yard mark, it is offset by about 9.52 fps (you need to add that amount to the Shooting Chrony Beta to match the velocity of the Caldwell); at the 1.5 yard mark, that drops to about 7.34 fps. I plan to use the Caldwell unit for the near distance (because recording the data is so super easy with the cord and app!), so I'm trying to find the offset to add to the Shooting Chrony unit to give me the 'Caldwell' equivalent velocity. I have been deliberating between using one or the other of those offset numbers, the average, or something else-and thinking myself into circles in the process. To make matters worse, the initial data may have been skewed by me inadvertently placing the Caldwell unit 37" from the muzzle (instead of the intended 36") due to the very light tripod getting bumped... The more I think about this though, the more I think I need to find the place where the extreme spread and STD stop changing signficantly as I move away from the muzzle-and then find the true 'offset' so that I know there isn't pellet stabilization phenomenon affecting the offset-and the resulting difference is truly the difference in chrony readings. Indeed, I think this is what I will need to do-so more testing will need to take place. Stay tuned.
3) As a side note: the Caldwell data download simply works and saves a LOT of time! I really like it. If you know the pellets's BC, the weight, the distance (in feet) from the muzzle, etc the app corrects for the distance you place the chrony away from the muzzle and tells you the true muzzle velocity, muzzle energy, etc. It also automatically pulls the weather conditions and logs those in the process-it's really slick and a real time saver!
Conclusion:
Once I do a bit more testing at distance, I'll have more confidence in what offset to use between the two chronographs, and will add it here to this thread. Once I have an offset I believe to be accurate, I'll commence with further BC testing and will be able to do it with half the pellets shot, using half the time, and half the air! I look forward to that day, as the setups are quite tedious to perform with repeatability.
I also intend to map BC changes over temperature of my most used pellets too-to see how weather affects BC...
Closing commments:
Please feel free to chime in with your experiences, observations, and any errors you may see in my method, etc-I truly appreciate your feedback and hope to help provide useful content to the airgun community. As I've said in other threads: if you find this information helpful / useful, take a minute and just click the '+' and leave me a feedback-it's a small way to say 'thanks' to me and for some reason, it helps make me feel like the time I spend contributing this info to the community is worthwhile and helpful to others.
Sincerely,
Sean
Method:
To start off with, I decided to position the first chrony in my standard (1 yard) position, and the second one a set distance behind it (18", measured center of rear window to center of rear window). The first chrony was positioned so that the 'rear' sensor window measured 36" from the end of my moderator. The second unit measured an additional 18" behind the first (or 1.5 yards from the end of the moderator). From there, I emptied 1 full .25 caliber magazine (shooting JSB Heavy, Mk I pellets) across both chronographs, recording the data, shot by shot, from both of them (later downloading into a spreadsheet for comparison).
The first magazine full was shot with the Caldwell unit at 1 yard, and the Shooting Chrony unit at 1.5 yards. After that, I switched the sequence and placed the Shooting Chrony Beta unit in at 1 yard, and the Caldwell unit at 1.5 yards from the end of the moderator, and proceeded to shoot a magazine full of lead over both units, record, etc.
The Data:
Now for the interesting part-the data:
Observations:
So here are a few things that stand out to me:
1) The data shows that the pellets are not fully stabilized at 1 yard from the muzzle, as evidenced by both chronographs showing a much larger extreme spread and standard deviation when positions 1 yard from the muzzle, to 1.5 yards from the muzzle (16 or 17 fps @ 1 yard vs.. 10.7 or 10 fps @ 1.5 yards). This, to me is a very significant finding--and perhaps should provide some measure of comfort to many shooters-regardless of what gun, ammo, etc they are shooting-that their gun is shooting just fine. Too often I read about people complaining about higher standard deviations when checking out a new gun, yet the gun shoot excellent groups at 50 yards (being a RAW shooter, I've read this from RAW owners, but also from a bunch of FX owners, as well as many other brands). Perhaps, as the data here suggests, the variation really shouldn't be taken a too much of a concern until one is sufficiently far enough from the end of the gun to ensure the Extreme spread and STD have settled down to represent what the gun is actually doing (when the pellet is stabilized). Since I have just discovered this (for myself) today, I don't have the answer on the 'ideal' distance from the muzzle to locate the chronograph for readings that represent stabilized pellets (the barrel may have an impact on that distance as well as the pellet, perhaps?).
2) I'm not sure I've settled on the offset number to use between the two chronographs-but the Caldwell consistently does read higher than the Shooting Chrony Beta unit - by about 10 fps. From the data I've shown above, at the 1 yard mark, it is offset by about 9.52 fps (you need to add that amount to the Shooting Chrony Beta to match the velocity of the Caldwell); at the 1.5 yard mark, that drops to about 7.34 fps. I plan to use the Caldwell unit for the near distance (because recording the data is so super easy with the cord and app!), so I'm trying to find the offset to add to the Shooting Chrony unit to give me the 'Caldwell' equivalent velocity. I have been deliberating between using one or the other of those offset numbers, the average, or something else-and thinking myself into circles in the process. To make matters worse, the initial data may have been skewed by me inadvertently placing the Caldwell unit 37" from the muzzle (instead of the intended 36") due to the very light tripod getting bumped... The more I think about this though, the more I think I need to find the place where the extreme spread and STD stop changing signficantly as I move away from the muzzle-and then find the true 'offset' so that I know there isn't pellet stabilization phenomenon affecting the offset-and the resulting difference is truly the difference in chrony readings. Indeed, I think this is what I will need to do-so more testing will need to take place. Stay tuned.
3) As a side note: the Caldwell data download simply works and saves a LOT of time! I really like it. If you know the pellets's BC, the weight, the distance (in feet) from the muzzle, etc the app corrects for the distance you place the chrony away from the muzzle and tells you the true muzzle velocity, muzzle energy, etc. It also automatically pulls the weather conditions and logs those in the process-it's really slick and a real time saver!
Conclusion:
Once I do a bit more testing at distance, I'll have more confidence in what offset to use between the two chronographs, and will add it here to this thread. Once I have an offset I believe to be accurate, I'll commence with further BC testing and will be able to do it with half the pellets shot, using half the time, and half the air! I look forward to that day, as the setups are quite tedious to perform with repeatability.
I also intend to map BC changes over temperature of my most used pellets too-to see how weather affects BC...
Closing commments:
Please feel free to chime in with your experiences, observations, and any errors you may see in my method, etc-I truly appreciate your feedback and hope to help provide useful content to the airgun community. As I've said in other threads: if you find this information helpful / useful, take a minute and just click the '+' and leave me a feedback-it's a small way to say 'thanks' to me and for some reason, it helps make me feel like the time I spend contributing this info to the community is worthwhile and helpful to others.
Sincerely,
Sean