Thread: Seating Depth, Harmonics, and Accuracy

Just an observation from a few weeks ago; I noticed that 2 separate seating depths gave the same sort of accuracy with a 5 shot group. Both a 20 thou and then a 40 thou jump produced the same tight groupings. Now I can understand how this might happen through the harmonic flex of a barrel, but wondering if anyone else has experienced this

Originally Posted by Bos

Just an observation from a few weeks ago; I noticed that 2 separate seating depths gave the same sort of accuracy with a 5 shot group. Both a 20 thou and then a 40 thou jump produced the same tight groupings. Now I can understand how this might happen through the harmonic flex of a barrel, but wondering if anyone else has experienced this

Did a 30 thou or 50 thou jump give the same grouping.

For my target rifles - using Lapua and Berger projectiles - jump make little noticeable difference to the grouping.
For example:
One of my rifles has fired over 6500 rounds.
I have not adjusted the seating depth since the first load was worked up for that rifle.
With throat erosion the seating depth has steadily increased.
The rifle groups as well today as it did when it was first run in.

It may be that the load & projectile is a very forgiving combination.
It also shoots well in my other 2 target rifles for which it wasn't developed.

I've also had the same result with different powder loads for the same projectile and rifle combination.
Two loads, 2.3 grains apart that group as well as each other.

In my experience the barrel, action, projectile combination determine the results.
Some are very forgiving, grouping over a wide range, some aren't, some are very fussy.

Only tried 20, 30, and 40 thou. 20 and 40 were tight groups, 30 was consideraby larger

Originally Posted by Cyclops

Did a 30 thou or 50 thou jump give the same grouping.

For my target rifles - using Lapua and Berger projectiles - jump make little noticeable difference to the grouping.
For example:
One of my rifles has fired over 6500 rounds.
I have not adjusted the seating depth since the first load was worked up for that rifle.
With throat erosion the seating depth has steadily increased.
The rifle groups as well today as it did when it was first run in.

It may be that the load & projectile is a very forgiving combination.
It also shoots well in my other 2 target rifles for which it wasn't developed.

I've also had the same result with different powder loads for the same projectile and rifle combination.
Two loads, 2.3 grains apart that group as well as each other.

In my experience the barrel, action, projectile combination determine the results.
Some are very forgiving, grouping over a wide range, some aren't, some are very fussy.

Do you mean "With throat erosion the jump to the lands has steadily increased" ?

With rifles a lot of things aren't set in concrete and seating depth is one of them, "like a box of chocolates etc."

Yes Forest, we know that but I thought I'd mention it anyway.

This is all you need to know. Erik Cortina is a bit of a legend.

https://www.youtube.com/watch?v=oRXlCG9YZbQ

Originally Posted by Dermastor

This is all you need to know. Erik Cortina is a bit of a legend.

https://www.youtube.com/watch?v=oRXlCG9YZbQ

Yeah, and then he puts up another video around that time saying how he adjusted the seating depth of his rounds during the evening ready for the next day of the Texas Nats. He loaded long, as they most do, and then adjusted based on land advancement from the day's shoot just completed. If that's not chasing the lands what is?

There is a good recent discussion on this topic on Precisionreloading blog.

Shooters finding deeper seating giving reliable and consistent accuracy. Same principle used in factory ammo and the logic behind Weatherby’ long throats/freebore and perhaps Remington’s old very long throated bels.

Some US match shooters getting longer throats to achieve the same without deeper seating ...

Originally Posted by Bos

Only tried 20, 30, and 40 thou. 20 and 40 were tight groups, 30 was consideraby larger

Greetings Bos,
You mention groups from which I assume you are shooting multiple groups at each setting. If not then the large group at .030" may just be due to chance and re shooting the series could give quite different results. Being a bit of a tightwad I use a standard jump to the rifling of 1mm assuming the rifle is capable of it. If this shoots well, and it usually does, I stop right there preferring to save the components for other uses. If however testing fills your heart with joy go to it but just remember one tight group does not guarantee an accurate load. Something it took me a while to learn.
Regards Grandpamac.