Seating depth

[Bottom Fodder]

Most seam to agree that seating depth has a major impact on accuracy and I’m not doubting that. I’ve seen it improve accuracy several time. My simple question is why does it? How does it? What’s your theory?

 

[gunsandgunsmithing]

It alters pressure and exit timing.

 

[6ShotsOr5?]

 

[Straightshooter1]

Most seam to agree that seating depth has a major impact on accuracy and I’m not doubting that. I’ve seen it improve accuracy several time. My simple question is why does it? How does it? What’s your theory?

It changes the case volume and the way the powder burns and also changes the volume in in the chamber that the blowby fills before the bullet is fully engraved into the bore (see graph below to get some idea how the pressure can change relative to seating depth). This results in the following effecting "precision":

It alters pressure and exit timing.

This ^^^^

 

[Bottom Fodder]

So when you do your test how many thousands for each step do you add?

 

[L.Sherm]

I dont add any, mine are hunting rifles so I start .010 past touch and work backwards .003 at a time. I also do it at 600 or farther.
On my 17 and small 20's I do at 100.

 

[ELR LVR]

It alters pressure and exit timing.

The higher Focus is bullet exit timing.
This is sensitive to within .005" easy
I run my tests moving .005" back at a time
You will see a pattern develop
Groups will start to close up then widen back out
There can also be 2 seating depths that work, so dont be afraid to keep going further
running a test going as far as 0.140" from jam
I have seen groups of 1" even when being .005" off the Node so to speak
Then grouping down in the .2's when you are on
Fine tuning is in the .001"
--------------------------------------
I included a pic of a powder "Ladder" test I did on a 22-250 just to show what it looks like when running a seating depth test
The pattern looks basically the same, it'll close up tight then widen back out

 

[Bill Norris]

So when you do your test how many thousands for each step do you add?

I find it best to only move the bullet .002 per increment as it seems the depth node in most cases is app. 006 wide.

 

[ELR LVR]

I find it best to only move the bullet .002 per increment as it seems the depth node in most cases is app. 006 wide.

I agree with this
.006" - .010" wide

 

[ELR LVR]

For long range Benchrest application, I’ve learned that optimum seating depth windows can be as small as .001, of course we start at .003 or so and narrow down using a few different formats or methods.
I like three shots per depth myself using different colored bullet tip. Myself I want not only the depth I’m set at to be stable but the depth on each side stable.. if you get my drift.
Anyway ~
I’ll post something sent to me by a friend that I thought was very interesting. This is a rough in seating test at long range, I’ve never really tried it myself but maybe one day just for drill, and before y’all say …that’ll never work. think again and maybe it will because the intent is to have stable exit timing.

Jim

To save on components, I have ran tests similar to this.
1 bullet of each difference of change in (whatever) be it depth or charge
And then noted where majority of them cluster
Then work from there

 

[Bill Norris]

For long range Benchrest application, I’ve learned that optimum seating depth windows can be as small as .001, of course we start at .003 or so and narrow down using a few different formats or methods.
I like three shots per depth myself using different colored bullet tip. Myself I want not only the depth I’m set at to be stable but the depth on each side stable.. if you get my drift.
Anyway ~
I’ll post something sent to me by a friend that I thought was very interesting. This is a rough in seating test at long range, I’ve never really tried it myself but maybe one day just for drill, and before y’all say …that’ll never work. think again and maybe it will because the intent is to have stable exit timing.
Shot at iirc 900 yards
Jim

But how do you factor in wind and/or conditions at that distance? Test like these I don't understand.

 

[Jeff Porter]

Do you think you will get different answers than you did in July ?

My thoughts are bullet exit timing and pressure impulse on the rifle system creating the vibration frequency are slightly altered by changing searing depth. The sweet spots will vary in location ( seating depth) from rifle to rifle , bullet weights and types, powder used and even primer used.

I will screen with 0.005” increments or some times even larger 0.030” spread on new combinations to get a feel for where a sweet spot may be. Then work deeper and longer in finer increments to get the tightest group and find the limits of the sweet spots.

 

[tom]

I believe seating to be more relationship/interface dependant than it is timing. Just based on different experiences over the years anyway.


The one thing that is a proven fact, is that it needs to be right to get the most out of the rifle.

Tom

 

[ELR LVR]

Do you think you will get different answers than you did in July ?

My thoughts are bullet exit timing and pressure impulse on the rifle system creating the vibration frequency are slightly altered by changing searing depth. The sweet spots will vary in location ( seating depth) from rifle to rifle , bullet weights and types, powder used and even primer used.

I will screen with 0.005” increments or some times even larger 0.030” spread on new combinations to get a feel for where a sweet spot may be. Then work deeper and longer in finer increments to get the tightest group and find the limits of the sweet spots.

Many people do not believe how much a primer change can make the last bit of a BIG difference and keep fighting their loads.
Once I find what i THINK, is my magic load, I then try every brand of primer I have.
It Always..............shows improvement or change
They Dictate what Powder/ Primer combo their rifle is gonna shoot, and like it!
Thats like dictating to your wife

 

[DShortt]

Lord.

Ya'll make my head hurt.

Seating depths tested to .001"

Ambient temperatures.

Air density.

Primer changes.

Case volume.

Neck tension.

BTO measurements.

Bullet weights.

Primer seating depths.



This has to be the most OCD group I'll ever see.

Have you ever considered how much money others are making off your perceived causes for failure?

It's a lot.

An auditor could retire attempting to unravel this.

It never ends.

 

[ELR LVR]

Lord.


This has to be the most OCD group I'll ever see.

That is a correct statement
(Now,
back to inspecting a trigger with a 30x magnifying glass looking for any imperfection
and hand winding return springs with Silver Plated Beryllium Copper wire)

 

[JFrank]

This has to be the most OCD group I'll ever see.

Just to point to how it can get. I recall someone soaking primers in some kind of liquid to dissolve the compound before re weighing them. @tom might be able to confirm that. lol

 

[ELR LVR]

Just to point to how it can get. I recall someone soaking primers in some kind of liquid to dissolve the compound before re weighing them. @tom might be able to confirm that. lol

He made them heavier before re weighing them?
Was he doing a Specific Gravity test?

 

[tom]

Just to point to how it can get. I recall someone soaking primers in some kind of liquid to dissolve the compound before re weighing them. @tom might be able to confirm that. lol

Allegedly lol.

I was killing them with water. The yellow bang bang sauce comes out, and makes "lemonade". It was about arguing over what it is you're weighing when you sort them.

Tom