Seating depth theory

[Bottom Fodder]

A lot talk and discussion on seating depth and a lot of theory about what all its doing to help with accuracy. We all agree “I think” that it does help

I would like to hear your thoughts on WHY Over the years I hear many theories from bullet alignment, adjusting case capacity, pressure and others. talking about this could enlighten us all

What’s your thoughts?

 

[6ShotsOr5?]

 

[ShtrRdy]

I would sure like to know why seating depth can make such a big difference. I've experienced it so I know it's real.

 

[jimmymac]

I don’t know the why or even care to know why, but for me seating depth is king and powder charge is queen in my quest for zero inch groups.

 

[Murray Brook]

I don't really think that there is a why. Some people prefer pizza others burgers. I've have rifles that shoot their best at touch and one that shoots great at only .120 off. Just preference I guess.

 

[357Mag]

Bottom -

Howdy !

Reloading reminds me of those " gaming " dice you see. It's " multi-faceted ".

Under the best circumstances, the shooter / re-loader up front has chosen the case, made an informed powder choice, has a pre-chosen bullet in mind; and ideally knows his preferred primer.....to kickoff the reloading process. If not, additional range testing will take place; to identify nail down the major cartridge components.

Once the prefered powder charge becomes known, fine scale load refinement via use of things like neck wall thickness & different neck tension ( via sizing ), any primer changes; and trying different seating depths takes place.... w/ focus on finding the most-accurate load. * In some circumstances & conditions, the shooter / reloader may have to " tune " the load via a powder charge change; to help keep accuracy ( and sometimes even load pressure ) at the desired level.

Range testing should reveal the best seating depth (accuracy wise ) for the chosen bullet...in the gun selected for test / use.

As regards bullet " jam ":
As a practical matter, there is only so much jam one can do, before either the bullet gets pushed excessively further down into the case, bolt closing force is negatively impacted; or perhaps the case becomes deformed.

Finding a great seating depth for your load, is one of the many fun aspects of shooting / reloading.


With regards,
357Mag

 

[Jim Ziegler]

If you seat the bullet deeper or more shallow, it affects the barrel exit timing.

 

[Rat Rifle]

My theory, changing the seating depth, changes the case volume, which changes the pressure, which changes the tune. Its that simple.

Those who have done an effective seating depth test, know it is a real thing.

 

[tom]

I'm of the opinion it is an "engagement relationship" thing. A few things point to that as the common denominator...

Examples,

1. 3 to 10 (for example)in works well with the same lead angle and bullet, despite the freebore length.

2. I had a wreck one match with a plastic jag, and ended up scoring a barrel an inch from the muzzle. Cut 2 inches off that night, shot a 4.xxx/100 the next morning. Re-tuned the rifle before the next match, and wound up not changing anything.

#2 actually also points to us tuning by pressure more so than a specific speed, but that's a different conversation.

Tom

 

[Bill Norris]

To quote Wayne Campbell on the podcast "Believe the Target" recently. Seating depth is the Most Important. Even more important than the powder charge.

 

[Bill Norris]

My theory, changing the seating depth, changes the case volume, which changes the pressure, which changes the tune. Its that simple.

Those who have done an effective seating depth test, know it is a real thing.

Agree 100%. Not a real fan boy of it in every situation but if you use the chrono across the seating depth test it clearly shows up.

 

[bugman]

The effect on engagement time ...or the effect on case capacity....take your pick.

 

[mikecr]

I don't believe seating depth is a tuning option, but instead a coarse prerequisite to tuning.
I believe that the function of seating adjustments to result amounts to a bullet-bore interface.
That is, finding an optimum bullet-bore interface.

To picture this, think of ping pong balls interfacing with a lotto exit tube. Some zip right into the tube,, some rattle around the mouth of the tube a bit. I believe there are optimum distances to release the ball from the tube.

Even though coarse seating results are independent of barrel or powder tune, changing seating depth affects tune. And it can move tune to a place that is not the best -in potential. That is why you should never do full seating testing from a tune. You should do it before tuning, as a prerequisite.

 

[Alex Wheeler]

I feel very fortunate to have been able to live in the pre internet era for a short time. Back then, in all the books and magazines this was very well known because people had to go out and do it for themselves. The precision shooting magazines and such. The idea this doesnt matter or that its not that important is very new and totally wrong. But to answer the question, its definitely a relationship to the lead. When I can chamber 100s or even 1000s of barrels in one cartridge and 90% of the guys end up within a couple thou of seating depth that tells the story. Its not capacity or distance to muzzle. I can use different freebores and the distance to lead stays the same. The barrels are cut to length with a tape measure, not critical. Doesnt matter. Now why does that relationship matter? My guess is the way the pressure curve forms. The pressure curve is important. Its why some powders shoot and others dont. Things like seating depth, primer, and neck tension all effect that curve.

 

[Jefferson]

To quote Wayne Campbell on the podcast "Believe the Target" recently. Seating depth is the Most Important. Even more important than the powder charge.

and of course

CHANGE THE SEATING DEPTH LAST IN THE PROCESS OF FINDING THE LOAD YOU WISH TO,USE OR START WITH ON COMPETING

 

[mikecr]

Fine tuning, or group shaping, is NOT full seating testing.
Whole different animal, and this is important.

For decades competitors claimed that VLDs had to be in the lands (ITL) to shoot well.
Turned out none of them ever did full seating testing. Not one of them..
They just made it up, and they were dead wrong.

 

[misser]

whining in the wind

 

[Alex Wheeler]

Fine tuning, or group shaping, is NOT full seating testing.
Whole different animal, and this is important.

For decades competitors claimed that VLDs had to be in the lands (ITL) to shoot well.
Turned out none of them ever did full seating testing. Not one of them..
They just made it up, and they were dead wrong.

They were not totally wrong though. All bullets have multiple places they will shoot well, usually 2 places in the lands and even more than that off the lands. But one of those places will out agg the others. Most of the time its in the lands. Thats not to say that theres not a jump or even a big jump that may be great or even more forgiving or have a wider window. But most of the real small aggs are in.

 

[Coyotefurharvester]

What came first tuning by seating depth or cutting the barrel off incrementally to tune for a specific load?

 

[Alex Wheeler]

What came first tuning by seating depth or cutting the barrel off incrementally to tune for a specific load?

Doesnt have anything to do with barrel length. I can think of many cartridges I chamber in many barrel lengths and those customers end up in the same place with seating and powder. The load really has to do mostly with the barrel maker and reamer. Keep those two the same and not much will change.