Inconsistent seating depth

[Baron23]

I’m going to ask this again....what is the practical concern. He has .004” max CBTO spread in one group and .002” max spread in another.

The resolution of his determination of jump distance is “15-20 thousandths”.

Why the fuck would anybody care about these very small differences in seating depth except as an academic exercise.


Am I missing something here. Diff in CBTO was .002”, right? and this is concerning? I believe a human hair is about 3-7 thousands and the throat is supposedly eroding more than .002” for every 100 rounds. I’m not trying to be snarky, just trying to understand the import of .002” of CBTO to any practical consideration. cheers

 

[918v]

I think the OP was concerned about the difference in consistency between the two reloading processes. From my work with the 6.5 CM I can tell you that .002” is nothing. The seating depth node of my current load is ten times more generous.

 

[Oldmauser]

I conducted the following test today at the range.

19 degrees Celsius, 92% humidity, headwind 13 km/h, 100 m off bench, F-class bipod, SEB rear bag.

I was shooting 140 gr Berger Hybrid Targets seated 15-20 thau off the lands, N 555 VV powder, Lapua brass.

I was shooting 5-shot groups. The first group consisted of ammo prepared using Redding body die+Redding neck sizing die. Neck was expanded using standard 21st century carbide mandrel. Bullets seated using Forster micrometer seating die.

Differences in the CBTO in this group were up to 4 thau.
Here is the group. There were 60 s pauses between each shot.


After that the barrel cooled down for 7 minutes.

Then I shot the group no. 2. This group consisted of ammo prepared using Forster FL die without a rod, then neck expanded by standard 21st century carbide mandrel. Bullets seated using Forster micrometer seating die. Differences in the CBTO in this group were up to 2 thau. It’s a 6 shot group.

So, here is the group no. 2. Quite disappointing.


I was not sure whether it was me, the rifle or the ammo. So I decided to repeat the group no. 2 after letting the barrel to cool down for about 8 minutes.

So, here is the repeated group no. 2.

 

[Oldmauser]

goofer dust , black magic need to sacrifice a chicken or two
View attachment 7661039
just to ask have you trimmed your cases ? or at least checked the length on all the cases and the bullets ?good luck figureing out what your problem is at least with the chicken you can eat it while you figure out what ever it is .

Cases were trimmed, deburred, chamf. Bullets- no, they were not sorted based on ogive or weight.

 

[Oldmauser]

That wasn’t the point.

If he is trying to measure cbto of 2.215” and his bullets induce a variance of +\-.001” then he will end up with 2.214” 2.215” and 2.216” long cartridges, actually.

If he takes his piece of shit caliper and measures the shortest round it may give a 2.213” reading or 2.215” reading. The longest reading could be 2.217” or 2.215”. Or all of them could read 2.215”.

It could very well be that he does not have any difference between his two groups.

It's a crucial factor you mentioned. I do have a quality caliper but yes there is a chance of error either tool's error or human error. This is why precision reloading is so difficult. If you have some thoughts on excluding such errors pls kindly share with us.

 

[nn8734]

It's a crucial factor you mentioned. I do have a quality caliper but yes there is a chance of error either tool's error or human error. This is why precision reloading is so difficult. If you have some thoughts on excluding such errors pls kindly share with us.

Your calipers are fine. Four thousands variation won’t make a difference - look no further than your own targets above. That second group looks more like shooter induced error than anything else, especially considering you put all five in one ragged hole when you shot group two again.

I don’t start to see issues on paper until about 15-20 thousands variation. This across all my CBTO variance testing over the years.

Overall, things look good on target. Time to test at 300 and 600.

 

[918v]

It's a crucial factor you mentioned. I do have a quality caliper but yes there is a chance of error either tool's error or human error. This is why precision reloading is so difficult. If you have some thoughts on excluding such errors pls kindly share with us.

If you want to know the actual numbers:

1. Get certified calipers. They measure 2.215” as 2.215” and not 2.214” or 2.216”. Your “quality” calipers are only +\-.001” accurate unless certified.

2. Make sure you don’t have ejector marks on the bottom of the case as they add a couple thou to the reading depending on the orientation.

3. Spin the round inside the comparator as you add pressure with the caliper jaws so the case settles down and sits square on the lower jaw.

4. I use a Sinclair hex comparator. The benefit is I know for sure that it sits squarely on the caliper jaws. The ones that bolt to the upper jaw are suspect as you can’t be sure they’re square.

5. Bullets are not perfect. There is going to be a little variance inside the lot. But like we told you it doesn’t matter for accuracy.

6. Keeping the above in mind, make your seating depth adjustments and records using the micrometer on top of the seater die and don’t use the comparator to chase exact numbers. So instead of writing 2.215” cbto on the case or on paper, use something like “Seater 4.005” or whatever and make .005” adjustments to fine tune.

 

[Deleted member 10043]

I’m going to ask this again....what is the practical concern. He has .004” max CBTO spread in one group and .002” max spread in another.

The resolution of his determination of jump distance is “15-20 thousandths”.

Why the fuck would anybody care about these very small differences in seating depth except as an academic exercise.


Am I missing something here. Diff in CBTO was .002”, right? and this is concerning? I believe a human hair is about 3-7 thousands and the throat is supposedly eroding more than .002” for every 100 rounds. I’m not trying to be snarky, just trying to understand the import of .002” of CBTO to any practical consideration. cheers

There is no where close to an academic excersise. It is the mental day room arguing over what to watch on the TV because Nurse Ratchet is busy taking a shit.

 

[Threadcutter308]

There is no where close to an academic excersise. It is the mental day room arguing over what to watch on the TV because Nurse Ratchet is busy taking a shit.

 

[smoothy8500]

I conducted the following test today at the range.

I was not sure whether it was me, the rifle or the ammo. So I decided to repeat the group no. 2

So, here is the repeated group no. 2.

So which is the real result or a fluke?

 

[Snuby642]

I'm going to toss this out.
I don't make a big practice of name dropping but I like the simple methods and explinations provided.

I have been testing his methods mostly on 223 ar rounds.
We all know that it's a poor test sled, that's a given.

That being said, it's a cheap test sled and I have seen the results since I can run a lot of tests.

Tests from a bolt gun with a good barrel and premium bullets will only magnify results.

His method is run full length cbto less a safety margin for powder.
Then once ocw is established start backing down lenght in 0.003 thousands incraments till your sweet spot is found.

His premise is at 5 or 10 thousands incraments you will likely miss the best spot.

My tests at 10 thousands incraments missed my spot at 0.006 that was best for that load.

When I saw results on paper I could hardly believe it was possible to show up on an ar.

You will always have + / - variables, cull them if you wish.

erik cortina reloading videos - Google Search

 

[Oldmauser]

 

[Snuby642]

Here's your sign

 

[Oldmauser]

i have this t-shirt and wear it during matches. lol.