I see the posts "help with OCW" or "what does this OCW test tell me?" Come up a lot. I started using this method a little while back, and I once called interpreting OCW results "reading the teal leaves." Dan Newberry developed this great methodology, but to interpret the results, people have been going to his website and asking the man himself. That is hardly efficient... or objective for that matter.
I have developed a methodology for determining accuracy nodes from OCW tests using two simple and widely available tools: On Target and Microsoft Excel.
I also do OCW a little differently in that I chronograph each shot. That gives me a separate set of data to work with. Here is how I do it:
1) I run an OCW test as can be found on Dan Newberry's site: http://optimalchargeweight.embarqspace.com/
When I am running my OCW, I will write the charge weight with a Sharpie on each round and divide the rounds up into groups of 3. When I chrony, I write down the shot number, the charge weight, and the velocity. My CED allows me to thumb back through velocities if I made a mistake (I have done that before, writing "1375" as opposed to "1735" or something like that).
2) I run the results on On Target, identifying not only each hole, but also using the "Point of Aim" command so that the relative vertical of each group is identified.
3) I plot out the average velocities for each charge weight in Excel. You can use the "average" function and select the three cells that contain the velocities for each charge weight. To do this, I make a column containing each of the charge weights used and next to each charge weight, I write "=average(" and select a cell, type ",", and select the next cell. You should have 3 velocities for each charge weight except for the first two charge weights and the last two.
ADVANCED:
If you are a real Excel monkey, then you might use the "averageif" function, and type "=avergeif(" and select the column with all of the charge weights, then ",", then select the charge weight you are needing the average of, then ",", and finally select the column with all of the velocities. Put a "$" in front of all of the column and row references except for the charge weights next to the column you are making. You should then be able to drag down to fill the column in. It is a little more complicated, but less work in the long run, and less error prone.
To create a plot, select the colums with charge weights and average velocities and go to the "insert" ribbon and select "scatter" and select the scatter chart with lines in between the points. This should make a nice graph of charge weight vs velocity. Ideally, you want a point where you added powder and didn't get much more velocity out. I know some people say this is a sign of max pressure or overpressure, but I have seen it at high, low, and moderate pressures.To me, it is something to exploit because a little overthrow or an underthrow, or any other kind of variation will not result in more or less velocity. That is a good thing.
4) I also create a new column next to the charge weight and velocity column that I call the "difference" column. I subtract the velocity in that row from the velocity in the previous row. What that tells me is where my smallest velocity jump happens. This is just a numerical depiction of the graph I just created. You can skip this step if you have done the previous step or vice versa. I like to do them both.
5) I make 3 columns in a new tab to interpret my OCW results: "Charge Weight," "Group Size," and "Off-set." I put all of my charge weights under the charge weight column. I put all of the 3-shot group sizes from On Target in the "Group Size" column, and then I put all of the "Group Offset Vert" from On Target in the third column. Then I make a fourth column called "Difference" and I subtract the previous "Group Offset Vert" from the "Group Offset Vert" in the cell I am working in. This will identify the change in vertical with each new charge weight. The goal of OCW is not to identify the smallest group (which is something I see people doing time and time again), it is to identify the smallest change in vertical from group to group.
6) FINDING THE ANSWER
We will find the smallest changes of vertical using the "Difference" column we just made. These are the accuracy nodes. I will then go back to the velocity graphs we made and identify the smallest "Difference" values there. If we have a correspondence of nodes between both OCW and velocities, then we have identified a good node.
According to OCW theory, we also want to avoid the "scatter node." In my example, the scatter node is 47.1 gr. My velocity node shows up somewhere between 47.1 and 47.4 gr, but my OCW node is somewhere between 47.4 and 47.7 gr. Therefore, I choose a charge weight of 47.4 gr. I also choose 47.4 gr because I want to stay a bit above 47.1 gr.
We aren't through. If you are shooting long range, you need to identify whether or not the node you have chosen will keep you above Mach 1.1 for the given range. For instance, for F-class, you would choose 1,000 yards and be sure that you are at least 10% faster than the speed of sound using JBM for the chosen velocity. Of course, the whole time, you are also looking for pressure signs to insure that you are not overpressure for a given node.
48.0 gr looks like a good node for me, but I am seeing slight pressure signs. For me, the extra fps isn't worth the wear and tear on the barrel and the risk to life and limb, but others might select that node. I might even select it if I were working it up in 110* F weather, but at 75* F, I don't want to risk the incremental temperature effects.
I have developed a methodology for determining accuracy nodes from OCW tests using two simple and widely available tools: On Target and Microsoft Excel.
I also do OCW a little differently in that I chronograph each shot. That gives me a separate set of data to work with. Here is how I do it:
1) I run an OCW test as can be found on Dan Newberry's site: http://optimalchargeweight.embarqspace.com/
When I am running my OCW, I will write the charge weight with a Sharpie on each round and divide the rounds up into groups of 3. When I chrony, I write down the shot number, the charge weight, and the velocity. My CED allows me to thumb back through velocities if I made a mistake (I have done that before, writing "1375" as opposed to "1735" or something like that).
2) I run the results on On Target, identifying not only each hole, but also using the "Point of Aim" command so that the relative vertical of each group is identified.
3) I plot out the average velocities for each charge weight in Excel. You can use the "average" function and select the three cells that contain the velocities for each charge weight. To do this, I make a column containing each of the charge weights used and next to each charge weight, I write "=average(" and select a cell, type ",", and select the next cell. You should have 3 velocities for each charge weight except for the first two charge weights and the last two.
ADVANCED:
If you are a real Excel monkey, then you might use the "averageif" function, and type "=avergeif(" and select the column with all of the charge weights, then ",", then select the charge weight you are needing the average of, then ",", and finally select the column with all of the velocities. Put a "$" in front of all of the column and row references except for the charge weights next to the column you are making. You should then be able to drag down to fill the column in. It is a little more complicated, but less work in the long run, and less error prone.
To create a plot, select the colums with charge weights and average velocities and go to the "insert" ribbon and select "scatter" and select the scatter chart with lines in between the points. This should make a nice graph of charge weight vs velocity. Ideally, you want a point where you added powder and didn't get much more velocity out. I know some people say this is a sign of max pressure or overpressure, but I have seen it at high, low, and moderate pressures.To me, it is something to exploit because a little overthrow or an underthrow, or any other kind of variation will not result in more or less velocity. That is a good thing.
4) I also create a new column next to the charge weight and velocity column that I call the "difference" column. I subtract the velocity in that row from the velocity in the previous row. What that tells me is where my smallest velocity jump happens. This is just a numerical depiction of the graph I just created. You can skip this step if you have done the previous step or vice versa. I like to do them both.
5) I make 3 columns in a new tab to interpret my OCW results: "Charge Weight," "Group Size," and "Off-set." I put all of my charge weights under the charge weight column. I put all of the 3-shot group sizes from On Target in the "Group Size" column, and then I put all of the "Group Offset Vert" from On Target in the third column. Then I make a fourth column called "Difference" and I subtract the previous "Group Offset Vert" from the "Group Offset Vert" in the cell I am working in. This will identify the change in vertical with each new charge weight. The goal of OCW is not to identify the smallest group (which is something I see people doing time and time again), it is to identify the smallest change in vertical from group to group.
6) FINDING THE ANSWER
We will find the smallest changes of vertical using the "Difference" column we just made. These are the accuracy nodes. I will then go back to the velocity graphs we made and identify the smallest "Difference" values there. If we have a correspondence of nodes between both OCW and velocities, then we have identified a good node.
According to OCW theory, we also want to avoid the "scatter node." In my example, the scatter node is 47.1 gr. My velocity node shows up somewhere between 47.1 and 47.4 gr, but my OCW node is somewhere between 47.4 and 47.7 gr. Therefore, I choose a charge weight of 47.4 gr. I also choose 47.4 gr because I want to stay a bit above 47.1 gr.
We aren't through. If you are shooting long range, you need to identify whether or not the node you have chosen will keep you above Mach 1.1 for the given range. For instance, for F-class, you would choose 1,000 yards and be sure that you are at least 10% faster than the speed of sound using JBM for the chosen velocity. Of course, the whole time, you are also looking for pressure signs to insure that you are not overpressure for a given node.
48.0 gr looks like a good node for me, but I am seeing slight pressure signs. For me, the extra fps isn't worth the wear and tear on the barrel and the risk to life and limb, but others might select that node. I might even select it if I were working it up in 110* F weather, but at 75* F, I don't want to risk the incremental temperature effects.
