My statement, in other words, do not look for a flat spot on the MV vs charge weight and expect to find the minimum vertical dispersion somewhere there.
Vertical Dispersion: Flat spots on the MV ladder test are meaningless
- Thread starter Beiruty
- Start date
rwj
Gold $$ Contributor
Anyone have access to a CFD (computational fluid dynamics) program to model the flow of high pressure gas thru an orifice? I’m thinking that the fluid (combustion gasses) ”see” the rifle barrel as a choked orifice. When an orifice is fully choked, an increase in upstream pressure will not result in a corresponding increase in flow rate (muzzle velocity).
standing waves
Aren't you looking for adjacent increasing linear loads?
Wouldn't this be no different than looking for a flat spot?
Flat spot or an increase in velocity could just be noise.
Both theories are looking at the velocity to tell the story
rwj
Gold $$ Contributor
Yup… something that the CFD analysis may predict!
I don’t know why you wouldn’t, that sounds like a stable area to me. What am I missing ?
Something’s going on, for sure. I haven’t hunted for V flat spots, ever, I’ll freely admit to that, but I do trust guys that use them to work up loads to be cognizant of the difference between real and merely aberrant flat spots.
It just seems that an accepted explanation would have been hashed out in times past. Usually that arises whenever a counterintuitive observation arises. We know that a combustion engine spinning too fast is actually producing less than peak power, but that is for understood reasons like insufficient time for complete burn, etc.
A thought I had is that, while we impede powder’s expansion, it actually could be stopped cold in a sufficiently strong chamber, leaving some “burned” with static pressure so high, that no more kernels will ignite because the inward pressure on each of them exceeds their expansive energy. (This would be like trying to fire a 55k PSI rifle cartridge submerged in a 100k PSI tank- or whatever number exceeds the final pressure of the impeded cartridge.) Going “partly there” with some charges, my thought was similar to yours, but I couldn’t see how that effect would be intermittent.
Last edited:
rwj
Gold $$ Contributor
It would no doubt be a challenge to model the combustion process, perhaps impossible with current programs (I’ve been retired for too long to be current). But a good start to asking the right questions and tweaking assumptions, would be just to model the transient flow of high pressure gas thru a small diameter tube. Kinda wonder if the DOD already has
!?CharlieNC
Gold $$ Contributor
I've done many charge ladder tests to typically find a wide node across several charge weights on the target whereby the poi on the target changes none to very little. Measuring velocity at the same time showed no similar behavior. Positive compensation clearly explains the target results with no relationship to velocity. Chase whichever you want!
The OPFS Theory would look for the minimum SD of MV for a group of 5 shots or more per load step.
That would guarantee the most stable MV for that load. Let us call this node, the OFPS node. However, due to harmonics of the barrel, minimum MV SD may not coincide with the OBT node, (almost 0 dilation/contraction of the barrel at the Muzzle)
In general, the next load step after OFPS is good compromise of the bullet dispersion and minimum SD of MV.
Note, the Jump of a given bullet can be tailored to minimize the radial dispersion of the bullets due to the bullet construction and bullet profile.
Load development I use:
In physics, there is no free lunch. In closed thermodynamics systems such as the internal ballistic solution, the goal is to get:
1) A very low MV SD vs powder charge load
2) Small groups vs powder charge load
3) MV velocity stability vs powder temperature
4) MV velocity stability vs small powder variations
5) MV velocity stability vs primer choice
6) Low vertical dispersion at very long and extremely long ranges vs bullet selection
First, let us solve the issues related to goals that we can control pretty easily. More specifically, 3) 4) 5) and 6)
3) Use temperature stable powder such as IMR endurance, Hodgdon Extreme Powder, etc. There are many online tests that you can search for and find what powders are the most temperature stable (e.g., H4350)
4) Measure each powder drop to less than 0.1gr accuracy. It is easy to get to 0.02gr (1 extruded kernel) which is worth 0.3fps out of typical load
5) Make a small ladder test and see if the selected primers would produce a smoothly rising curve, if not, do not use those primers.
6) Use VLD bullets with very low variations in G7 BC. For example, pick VLD bullets with less than 1% variation in G7 BC. Now, here we finished the goals that are virtually 100% influenced by the components
Now, let us focus on the loading for the node. What is the node after all?
Optimum Barrel Time, OBT node: It is the node (with respect to time) where the bullet leaves the barrel when the muzzle is subjected to the least dilation/contraction due to the powder explosion shock waves
Optimum Charge Weight, OCW node: it is the node (with respect to time) where the bullet leaves the barrel when the muzzle is pointing to the same point of impact due to the powder explosion shock waves
Optimum Feet per sec, OFPS node: it is the node (with respect to time) where the bullet leaves the barrel with the minimum SD of MV due to the powder explosion shock waves keeping all parameters the same, changing the powder charge can change the MV and inversely the actual barrel time of the bullet
Note: OFPS is my theory why MV SD would shrink for the same powder but by just changing the powder charge. What node are we after?
Goal 1) is solved with OFPS node, a theory I introduced in September 2018.
Goal 2) is solved with OBT time. The good news, OBT is numerically computed. With the advancement of very accurate radar-based chronographs and internal ballistic solvers, the OBT charge load can be accurately calculated. I had success doing that for a few competitive shooters as proof of concept.
How about Goal 1) the best way to find it in my experience is to shoot a group of 3-5 shots in a ladder test such as the 6.5group spreadsheet test and find the lowest MV SD The bad news is OBT node (the smallest group) and OFPS node (the lowest MV SD) don't overlap. Remember, in Physics, there is no free lunch. The good news is both nodes are adjacent and in my experience, if you find the OBT node, the OFPS node is the next step load in the ladder. As groups shrink, MV SD will increase and vice versa. So, the best is to find a compromise load between OBT/OFPS nodes that works for the shooter.
In physics, there is no free lunch. In closed thermodynamics systems such as the internal ballistic solution, the goal is to get:
1) A very low MV SD vs powder charge load
2) Small groups vs powder charge load
3) MV velocity stability vs powder temperature
4) MV velocity stability vs small powder variations
5) MV velocity stability vs primer choice
6) Low vertical dispersion at very long and extremely long ranges vs bullet selection
First, let us solve the issues related to goals that we can control pretty easily. More specifically, 3) 4) 5) and 6)
3) Use temperature stable powder such as IMR endurance, Hodgdon Extreme Powder, etc. There are many online tests that you can search for and find what powders are the most temperature stable (e.g., H4350)
4) Measure each powder drop to less than 0.1gr accuracy. It is easy to get to 0.02gr (1 extruded kernel) which is worth 0.3fps out of typical load
5) Make a small ladder test and see if the selected primers would produce a smoothly rising curve, if not, do not use those primers.
6) Use VLD bullets with very low variations in G7 BC. For example, pick VLD bullets with less than 1% variation in G7 BC. Now, here we finished the goals that are virtually 100% influenced by the components
Now, let us focus on the loading for the node. What is the node after all?
Optimum Barrel Time, OBT node: It is the node (with respect to time) where the bullet leaves the barrel when the muzzle is subjected to the least dilation/contraction due to the powder explosion shock waves
Optimum Charge Weight, OCW node: it is the node (with respect to time) where the bullet leaves the barrel when the muzzle is pointing to the same point of impact due to the powder explosion shock waves
Optimum Feet per sec, OFPS node: it is the node (with respect to time) where the bullet leaves the barrel with the minimum SD of MV due to the powder explosion shock waves keeping all parameters the same, changing the powder charge can change the MV and inversely the actual barrel time of the bullet
Note: OFPS is my theory why MV SD would shrink for the same powder but by just changing the powder charge. What node are we after?
Goal 1) is solved with OFPS node, a theory I introduced in September 2018.
Goal 2) is solved with OBT time. The good news, OBT is numerically computed. With the advancement of very accurate radar-based chronographs and internal ballistic solvers, the OBT charge load can be accurately calculated. I had success doing that for a few competitive shooters as proof of concept.
How about Goal 1) the best way to find it in my experience is to shoot a group of 3-5 shots in a ladder test such as the 6.5group spreadsheet test and find the lowest MV SD The bad news is OBT node (the smallest group) and OFPS node (the lowest MV SD) don't overlap. Remember, in Physics, there is no free lunch. The good news is both nodes are adjacent and in my experience, if you find the OBT node, the OFPS node is the next step load in the ladder. As groups shrink, MV SD will increase and vice versa. So, the best is to find a compromise load between OBT/OFPS nodes that works for the shooter.
@Beiruty, what is your experience level. What have you won? There are some incredible shooters on this site (far better than myself by the way) offering you first hand results of performance; however, you just keep talking about Hornady’s 4DOF results. Sometimes listening to what others have to say with an open mind is the best way to improve your skillset.
Dave
I think the word you are looking for is POP QUIZ.
Love the dumpster fire. I totally ripped that pic to send to my buddies the next time it’s appropriate
PLEASE don’t take this the wrong way, but MANY MANY times in my development of various loads for many different rifles, the load with the smallest MV SD is NOT the most accurate load.
Dave
LVLAaron
Gold $$ Contributor
at what distance?
Thank YOU. This is my approach as well and it has never let me down. When you combine this method with the use of a tuner to identify the positive compensation point and then to keep it in that tune as temps vary.
Dave
At 100 and 200 yards. I do all of my load work at 100/200 yards. I’ve always found that a great load selection at 100 or 200 yards and it has translated fantastically to 600 and 1000 yards. I shoot F class though, NOT benchrest.
Dave
At our college, the term "Drop Quiz"
At our college, it was widely used among professors, and it meant drop everything you are doing, and start working on an instant quiz.
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