I've been involved in a non-shooting project over the winter and haven't shot or chronographed since last fall. Today I went out and fired a few rounds from my .338 Edge over my triplicate chronograph setup in preparation for a bear hunt. I neglected to bring the skyscreen shades for the Oehler 35P, and when the full sun popped out from behind the clouds, the 35P burped on two shots. Under cloud cover, no problems. The other two chronographs, an Oehler 33 and a PACT PC2, did both have their skyscreen shades mounted overhead. I only chronographed five (5) shots from the 338 Edge. Three (3) under cloud cover that resulted in the collection of good velocity data. Two (2) under full sun that caused some bad data from the Oehler 35P - due to the clear blue sunny sky overhead (no shades over the 35P skyscreens).
I'll provide the 3 rounds that yielded credible data. The first round was a 200 grain Speer bullet that I was using to bore sight the rifle. The next two bullets fired were 265 gr Henson Aluminum Tipped - Rebated Base Boattails.
Oehler 35 Primary 3145 2900 2908 > 6 foot skyscreen spacing
Oehler 35 Proof. . 3152 2906 2918 > 3 foot skyscreen spacing
Oehler 33 . . . . . 3146 2852
2909 > 6 foot skyscreen spacing
PACT PC2 . . . . . 3148 2903 2908 > 56 inch (4' 8") skyscreen spacing
fps from the 2nd shot recorded on my Oehler 33. Obviously the Oehler 33 burped on this 2nd shot and recorded a bogus velocity. But this 3-shot data set illustrates the value of having more than one chronograph recording each and every shot. If I'd only been using the Oehler 33, there would have been no way to know that it provided a value 51 fps slower than the PACT chronograph, 48 fps slower than the 35P primary channel, and 54 fps slower than the 35P proof channel. No way to figure out why the velocity was 48 to 54 feet slower than the velocity that should have been recorded.
After shooting these rounds from my .338 Edge, it was pretty clear that the Proof screens for the Oehler 35P were mounted too close together on my skyscreen rail, since the Proof velocities being recorded were 7, 6, and 10 fps faster than the velocities recorded on the Oehler 35P Primary skyscreen. The Proof skyscreens on the 35P are mounted 1/2 the separation distance of the primary skyscreens, or a 3' spacing. In addition, the 35P Primary channel velocities recorded were a dead match to the Oehler 33 and PACT chronograph data, so I knew the Proof skyscreens needed adjusted - not the Primary skyscreens.
So I came home and got the Ruger .22 semi-auto pistol out and shot in the back yard, in order to adjust the Oehler 35P proof skyscreen separation distance - to yield a Proof channel velocity which would better match the 35P Primary channel velocities. I increased the separation distance between the Proof skyscreens about 1/8". Here's the first 5 shots fired
, using a variety of .22LR ammo that I found (pickups from the gun range).
Oehler 35 Primary 1017 1046 1055 939 1038
Oehler 35 Proof. . 1019 1048 1057 940 1040
Oehler 33 . . . . . .1017 1045 1054 938 1037 > My Oehler 33
PACT PC2 . . . . . .1015 1045 1054 938 1037
The 35P Proof channel was still about 2 fps faster than the 35P Primary channel. And the 35P Primary channel velocities were about 1 fps faster than the Oehler 33 and PACT velocities. So I separated the 35P Proof skyscreens about 1/16" farther apart before firing the next five rounds. Next, I conducted a very interesting experiment
. I replaced my Oehler 33 chronograph with a different Oehler 33 that I'd purchased from LRH Forum Member 'Alan Griffith
' this past December. I'd never used Alan's Oehler 33 before, and I wanted to see if and how it worked, and how the data from Alan's Oehler 33 would compare with the data from my 33 when plugged into the same exact skyscreens. So here's the data from the next five shots
Oehler 35 Primary 1042 1050 1030 1037 1031
Oehler 35 Proof. . 1043 1052 1031 1037 1032
Oehler 33 . . . . . .1041 1050 1029 1036 1031 > Alan Griffith's Oehler 33
PACT PC2 . . . . . .1041 1050 1029 1036 1031
Dang! Alan's Oehler 33 continued to spit out data exactly matching the data from my PACT PC2. To be honest, I didn't expect to see exactly matching data from this second Oehler 33. After the 3rd of these 5 shots, I again slightly increased the spacing on the 35P Proof skyscreens, and on the 4th shot, you can see that the 35P Proof channel matched the 35P Primary channel velocity at 1037 fps.
Prior to firing the next three shots I increased the spacing of the Oehler 35P Primary skyscreens slightly. The Oehler 33s and the PACT PC2 had recorded the exact same velocities
for the past 9 shots fired. I wanted to see if I could get the 35P Primary channel to read 1fps slower to bring it into agreement with the two Oehler 33s and the PACT chronographs. Next three shots:
Oehler 35 Primary 920 1055 1079
Oehler 35 Proof. . 919 1054 1079
Oehler 33 . . . . . .920 1054 1078 > Alan's Oehler 33
PACT PC2 . . . . . .920 1054 1078
At this point the Oehler 33 and the PACT PC2 chronographs had recorded the same exact velocity for the prior 12 shots. Pretty remarkable! I fired three more rounds just to see how long this string of matching velocities would last. Here's the data from the last three shots fired:
Oehler 35 Primary 1072 1031 969
Oehler 35 Proof. . 1073 1030 969
Oehler 33 . . . . . .1071 1030 968 > Alan's Oehler 33
PACT PC2 . . . . . 1072 1026
As you can seen, the string of duplicate data from the Oehler 33 and the PACT chronographs stopped at 12 consecutive shots. The PACT chronograph produced a slightly errant value (1026 fps
) on the second of these last three shots. Not a gross error, but 4 fps slower than the Oehler 33 and 35P Proof channel. All three chronographs are yielding data that more or less match one another with my current skyscreen spacings.
Quite a few times over the years, I've read Posts that pooh poohed
the value of chronograph data for purposes of determining muzzle velocity. Some even claim measuring bullet drops over extended ranges is a better method of determining muzzle velocity. I clearly disagree with that camp of shooters. After reviewing the quality and consistency of the above data, y'all can make up your own mind on that topic. I do believe a shooter needs to have at least a tandem chronograph setup - two recorded velocity data for each bullet fired - in order to place high confidence in the data. Either for purposes of identifying the ES and SD of one's loads. Or for purposes of entering valid muzzle velocities in a ballistics program, such as LoadBase 3 or Exbal.
Hope this presentation of data is clear enough that you're all able to understand/interpret the data. Feel free to ask questions, if necessary. It was fun to get out and shoot and do a little more chronographing.