LaHunter
Well-Known Member
Eric Cortina has some good videos discussing this. His info is a good resource and his performance speaks for itself.
What “Causes” a Low SD/ES?
- Thread starter General RE LEE
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He didn't say he had an inconsistency, he asked how a change in powder charge can cause such a change in SD/ES.
Randy Tidwell
Well-Known Member
One of the best responses I've seen on here. ROFLMAO
Low ES / SD is caused by repeating exact same pressure curve with the exact same bullet shape / material / weight.
Things that help this
- Powder mass (repeatable down to the kernal)
- Brass internal volume (some do this by weight sorting brass w/ exact headspace)
- Consistent neck tension (crimp helps as well)
- Consistent ignition (uniform flash holes, primer seating depth)
There are other things but those help the "most". When you find a "node", you are now timing your bullet to exit the barrel at a maximum deflection (up or down). Think of a sine wave with the bullet exiting at peak or valley. Doing this provides forgiveness in being off in the other areas above because the duration of the barrel in that position is slightly longer than not in a node. This is most likely what you experienced by moving up 0.3 grains, you left a peak / valley.
Nodes are great, but I've put 10 rounds within 2 fps without being on a node but that costs a lot more money with expensive scales.
Things that help this
- Powder mass (repeatable down to the kernal)
- Brass internal volume (some do this by weight sorting brass w/ exact headspace)
- Consistent neck tension (crimp helps as well)
- Consistent ignition (uniform flash holes, primer seating depth)
There are other things but those help the "most". When you find a "node", you are now timing your bullet to exit the barrel at a maximum deflection (up or down). Think of a sine wave with the bullet exiting at peak or valley. Doing this provides forgiveness in being off in the other areas above because the duration of the barrel in that position is slightly longer than not in a node. This is most likely what you experienced by moving up 0.3 grains, you left a peak / valley.
Nodes are great, but I've put 10 rounds within 2 fps without being on a node but that costs a lot more money with expensive scales.
tony d willIiams
Well-Known Member
Time and data.
When I set up a load, everything remains the same. Even primers, I buy by the bricks, same lot numbers.
Small changes, one at a time. Including stand off, type of powder and all the rest. Any large changes on the range give pause, what happened?
My final set up is in the mid section, average of all bullets reaching distance, speed, and size of groups.
I did have one powder which was very finicky to temperature changes, very finicky. Now I carry my rounds next to my skin, 96℉, ±3℉.
Loose nut behind the wheel.
Have fun, be safe and enjoy yourself.
MagnumManiac
Well-Known Member
I forgot to mention a few other variables that can definitely cause fluctuation in start pressure; flash hole uniformity, primer seating crush uniformity, case volume (not weight) consistency and even neck length or square mouths.
Case volume is a big issue, if it's not the same across the batch then you will just be chasing your tail.
Unless all these variables can be seen on target by change, then the rifle/brass/powder system may not be capable of holding a wide node.
A wide node is what you need, sure a narrow node works, but as the OP discovered, a small change can disrupt the flow, so to speak.
Cheers.
Case volume is a big issue, if it's not the same across the batch then you will just be chasing your tail.
Unless all these variables can be seen on target by change, then the rifle/brass/powder system may not be capable of holding a wide node.
A wide node is what you need, sure a narrow node works, but as the OP discovered, a small change can disrupt the flow, so to speak.
Cheers.
Percentage of powder volume fill; high angle shooting......up versus down versus level.... with excess powder void shifting versus compressed powder loads; temperature variations.....rounds laying in sun versus shade. Rounds in gun at 0F versus rounds inside your parka at 98.6F. Even heavy vibration in ammo transport can contribute to powder packing/shift and ignition/burn variables. Throw your packed up ammo in a pickup bed, some ammo horizontal;some vertical; tip up/down, and drive 25 miles up a rocky rutted washboard road....powder may compact more/differently. So many variables and combinations of variables impact ignition/burn characteristics and thus SD/ES. As FEENIX stated....Consistency is crucial. CONSISTENCY is King...in reloading techniques, ammo handling, every phase of precision, extended range shooting.
Doom2
Well-Known Member
Based on the numbers you give I'm willing to bet you reference 3 shots in each sample. If that's the case the sd's are not statistically different.
MagnumManiac
Well-Known Member
Andrew Massi
Well-Known Member
I bet your "inconsistent" load with the terrible es of 20 is inside a deers vitals, or minute of venison, out past the terminal abilities of the bullet. Run the impact differences at 1,000 sometime and you'll spend less time reloading and more time shooting 
nksmfamjp
Well-Known Member
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My point was this looks mostly like a small sample size error….
Sorry….I think.LL! Now you've done it...
This is a good post. If you load more test loads around this area, you can get and idea of the node width by average velocity change per grain and the 5rd sd'. When you think you are close to center node, load 25 and shoot 5 groups of 5 with the same chrono group. How does Sd look now? Still under 10?
