Purpose of bullet jump

[mwkelso]

This question is for the experienced smiths and hand loaders.
When fine tuning a hand load's seating depth, what is actually being adjusted?
I know that different COAL's can affect chamber pressure, therefore affecting muzzle velocity. Is seating depth changing muzzle velocity ever so slightly to get the best timing of the bullet exiting the barrel at it's most natural point?
Or do the micro adjustments of .005-.010" make that much of a difference in the bullet's exit timing?

 

[Rhovee]

It effects the time a bullet leaves the barrel. If you do a seating depth test with a long horizontal board, you will see it form a sign wave. It then comes together and then starts to open up again. I usually find about .005 variation max on seating depth accuracy.

 

[mwkelso]

It effects the time a bullet leaves the barrel. If you do a seating depth test with a long horizontal board, you will see it form a sign wave. It then comes together and then starts to open up again. I usually find about .005 variation max on seating depth accuracy.

Is that +/- .005 variation in depth what your fine tuning is after you have determined the ideal charge weight for that cartridge?

 

[AZShooter]

That is what I do Kelso,

I fine tune seating depth after finding the best powder charge. I remember back in the 80s reading in Shooting Times by an author named Rick Jamison doing that approach. At the time most of the guys I knew at the range that handloaded thought he was nuts to mess with seating depth. Now it is common place.

FYI if you use Berger hybrid bullets there is a high probability you won't have to alter seating depth just work up the powder charge. I have used quite a few hybrids and I would say that is the norm.

 

[Mikecr]

I don't think seating tests indicate tuning at all.
The adjustments affect tune, but that don't mean they are actually a part of tune.
I think seating adjustments establish quality of bullet interfacing with the bore (better/worse).

With this, and witnessing seating adjustments having a huge affect to results(way beyond any powder adj), I consider FULL seating testing to be prerequisite to the tuning phase. Same with primer swapping (another prerequisite)(not tuning).
So I do this type of testing during fire forming of brass, and far away from any expected tune.

Now, before dismissing my notion on this, which is new to you I'm sure, keep in mind that truths pass ALL tests.
The prior discussions right here point out a failure in the standard seating tuning ideas.
For example:
-----------------------------------------------------------------------
If bullet seating is tuning, then why would the shape of a bullet nose (tangent/secant/hybrid) w/resp to land relationship, make any difference?
Wouldn't 20thou OTL mean the same to tune with any bullet nose shape?
-----------------------------------------------------------------------
In fact, the example chosen 20thou OTL does mean the same -to tune -for any ogive shape. Yet that relationship may be horrible for one ogive type, while great for another (in YOUR bore). The results can be different while tune is not different. This, even with utmost precision in tension and load density compensated for,, one kernel of powder at a time.
Therefore, good/bad results from differing ogive type & relationship with lands has nothing to do with tune.

Can anybody provide information indicating a failure of my notion?
If you can, in any regard, then I'm wrong.

 

[catamountsierra]

I don't think seating tests indicate tuning at all.
The adjustments affect tune, but that don't mean they are actually a part of tune.
I think seating adjustments establish quality of bullet interfacing with the bore (better/worse).

With this, and witnessing seating adjustments having a huge affect to results(way beyond any powder adj), I consider FULL seating testing to be prerequisite to the tuning phase. Same with primer swapping (another prerequisite)(not tuning).
So I do this type of testing during fire forming of brass, and far away from any expected tune.

Now, before dismissing my notion on this, which is new to you I'm sure, keep in mind that truths pass ALL tests.
The prior discussions right here point out a failure in the standard seating tuning ideas.
For example:
-----------------------------------------------------------------------
If bullet seating is tuning, then why would the shape of a bullet nose (tangent/secant/hybrid) w/resp to land relationship, make any difference?
Wouldn't 20thou OTL mean the same to tune with any bullet nose shape?
-----------------------------------------------------------------------
In fact, the example chosen 20thou OTL does mean the same -to tune -for any ogive shape. Yet that relationship may be horrible for one ogive type, while great for another (in YOUR bore). The results can be different while tune is not different. This, even with utmost precision in tension and load density compensated for,, one kernel of powder at a time.
Therefore, good/bad results from differing ogive type & relationship with lands has nothing to do with tune.

Can anybody provide information indicating a failure of my notion?
If you can, in any regard, then I'm wrong.

What you are saying makes a lot of sense to me. I've been kicking around a number of different questions about seating depths, largely because I never found a really good load for my Rem700 while it had it's factory 7mm-08 barrel on it. I played around with several bullets, a couple powders, and different seating depths, but never got it to shoot better than about a minute and a quarter; pretty much everything I tried shot at that level regardless, so at least it was consistent that way. Anyway, I sent it off to the gunsmith and it came back as a 284 Winchester. Right off the bat it shoots better, so now I want to squeeze it for all it is worth. I had been just fireforming my brass with cheap bullets and some powder that I just wanted to use up and planning on starting load development after the brass was formed, including seating depth testing.

Now, since I still have another couple hundred cases I can form for that rifle, I am thinking that I may be able to do seating depth testing while forming brass, even if I am using a different powder than I plan to use later for the actual load development. At least that makes sense if I am understanding your post correctly. I could see some fine tuning after powder charge testing, but it sounds like I wouldn't be shouldn't be starting from scratch if I change powder. Likewise, I think I should be able to do the seating depth testing for my new 223AI barrel while forming brass for it. I'm just hoping to confirm that I understand this correctly before I load up a bunch of more expensive bullets as a test that wouldn't actually carry over to my final load development.

 

[bigedp51]

QuickLOAD Review & User's Guide
https://www.6mmbr.com/Quickload.html

Key Features of QuickLOAD
QuickLOAD calculates and predicts how changes to these important variables affect your loads:

• Bullet Seating Depth
If you haven't tried QuickLOAD, you may be surprised at how much of an effect relatively small changes in seating depth can have. QuickLOAD gives you a good sense of how seating depth (and case capacity) affects load pressure.

 

[bob4]

I do seating tests while I'm fire forming.

 

[Lefty7mmstw]

I don't think seating tests indicate tuning at all.
The adjustments affect tune, but that don't mean they are actually a part of tune.
I think seating adjustments establish quality of bullet interfacing with the bore (better/worse).

With this, and witnessing seating adjustments having a huge affect to results(way beyond any powder adj), I consider FULL seating testing to be prerequisite to the tuning phase. Same with primer swapping (another prerequisite)(not tuning).
So I do this type of testing during fire forming of brass, and far away from any expected tune.

Now, before dismissing my notion on this, which is new to you I'm sure, keep in mind that truths pass ALL tests.
The prior discussions right here point out a failure in the standard seating tuning ideas.
For example:
-----------------------------------------------------------------------
If bullet seating is tuning, then why would the shape of a bullet nose (tangent/secant/hybrid) w/resp to land relationship, make any difference?
Wouldn't 20thou OTL mean the same to tune with any bullet nose shape?
-----------------------------------------------------------------------
In fact, the example chosen 20thou OTL does mean the same -to tune -for any ogive shape. Yet that relationship may be horrible for one ogive type, while great for another (in YOUR bore). The results can be different while tune is not different. This, even with utmost precision in tension and load density compensated for,, one kernel of powder at a time.
Therefore, good/bad results from differing ogive type & relationship with lands has nothing to do with tune.

Can anybody provide information indicating a failure of my notion?
If you can, in any regard, then I'm wrong.

Mikecr is entirely correct in his surmising that seating depth tuning is really effecting the interface between the barrel and bullet. How else would you be able to cheat at times and set the bullet into the lands for better accuracy. The nose slump, obduration, and amount of skidding allowed as the bullet couples with the rifling all change with seating depth changes.
All of this is much more pronounced with shooting cast bullets in rifles. I was lucky to find a cast bullet load for my 375h@h that is just over moa the first run up. I have yet to find an acceptable load with a cast pill in my 405 Winchester. I will likely end up using a card or poly wad under the bullet base to protect it. 2"+ groups at 25 yards aren't acceptable even for a buckhorn sights lever rifle...

 

[birdiemc]

It effects the time a bullet leaves the barrel. If you do a seating depth test with a long horizontal board, you will see it form a sign wave. It then comes together and then starts to open up again. I usually find about .005 variation max on seating depth accuracy.

Got any pictures? I'm picturing perhaps a 2x6 a couple feet long with a straight line drawn down the length of it centered top to bottom, aim at the line, then move over an inch, repeat...
Just 1 shot per bullet seating depth or groups? I'm having a hard time understanding what the sine wave is going to indicate? Maybe I just need to sleep on it...haha

 

[Rhovee]

Use a 2'x3' board or so and shoot groups. I usually load up around 30 rounds of sand powder charge. Bring my Arbor press and seating die to the range. I make them all .003 in the lands and work back. Seat and shoot two. If they are good shoot more. If not keep moving deeper. You all see them move up and down, then come together with Same poi.

 

[mwkelso]

@Mikecr
Just for my clarity- when you are developing a new load, do you start by adjusting seating depth and/or primers before adjusting your powder charge weight?
You're right, it is definitely a concept that I am not familiar with.

 

[Mikecr]

Yes, I do full seating and primer testing while fire forming brass (if new).
Brass needs to be fully fire formed to stable before moving on to powder.
By 'stable' I'm not just referring to static capacity but also to dynamic capacity, which has a significant affect to the shaping of your pressure peaks. Your timing.

When my brass is already FF'd, and I already know a best powder load for a bullet weight, but now testing different bullets or primers, I back off the charge -considerably away from any powder node for this testing.
I don't want a powder node collapsing/forming at the same time that I'm trying to assess seating or primer results.

Seating and primers are prerequisite. Case chamber fit & load density are coarse to results. Powder, neck tension, and final seating tweaking(within window) are fine to results.

Your powder testing is likely one or the other of two paths: a forgiving OCW, or cutting edge precision. It's rare to have both, but if you follow my order in this your chances are better.
You need to reach either before moving onward to accuracy, which is single shot adjustments from hot bore results to cold bore desire.
I've yet to see a load at best precision AND best accuracy at once.
It likely leads to a choice; one or the other, or good enough or not.

 

[BallisticsGuy]

I go in ~30-40thou increments just looking for a place that's better than the others in group size. In 4 increments set .030 apart I'll see a trend in much in the way that Rhovee describes but I will likely see only a continuous increase or decrease in group size because I don't try to find the point of absolute mininmum group size. I do not get bent around a rod trying things out at single digit thou increments and I don't jam unless it's really necessary but if I was doing bench rest competition I almost certainly would do at least one of those things. For 90% of people going to that small of an interval is just getting into the weeds.

What is actually the "reason"? Let me be 100% clear: I don't "know". So, SPECULATION BELOW.

Here's some speculation which seems backed up well by experimental data but I don't have enough data to say my observations are more than total coincidence. It's seems to be to do with having enough velocity built up between the time the bullet defeats the case mouth friction and the time it reaches the rifling origin to defeat engraving pressure without pressures having to climb excessively high. There are several events that happen when you fire a rifle. Primer detonation increase case pressure sharply which should dislodge the bullet from case neck clamping. While the powder conflagrates and pressures further build the bullet is jammed into the rifling by some amount (probably not a lot) until pressures build enough to finish the engraving process and overcome bore friction to send the pill the rest of the way down.

Long leades of especially low angle would probably want jammed bullets. They don't need a lot of inertia because the rifling comes up very gently so each unit of distance the bullet moves requires less force to get it there than a sharper rifling angle might net.

"Normal" heavy bullet optimized leades of average steepness should probably show a tendency to like a relatively relatively short but definite jump. Abrupt rifling angles would probably show a tendency to like much longer jumps so that enough inertia is built to engrave the bullet. Tangent vs. Secant ogive shapes probably play a rather significant role in that system too. Classic VLD's with the sharpish transition from shank to nose I can see making the whole works more sensitive to all kinds of little things based on the above reasoning.