Seating depth…what does it do?

[David Christian]

Been restudying a number of different items to include OBT, load development, neck tension, etc.

Curious on what this group thinks a change in seating depth does in fine tuning a load for precision.? Looking for all angles here. Thanks!

 

[Jeff Porter]

Barrel time- exit point in the vibration cycle of the barrel moved by a few milliseconds to find the node of best accuracy.

 

[BoydAllen]

When working up a load I start with a seating depth that has given good results in the past, test for powder charge and then when I have that go back and look at small changes in seating depth. IMO there is a lot of noise on the internet regarding working up loads, that makes people think that it has to be a lot more complicated than it has to be.

 

[K22]

I never found seating depth to be a significant element in developing accurate loads. I'm referring to the seating depths often reported in published reloading data for specific bullets, especially in the Sierra and Nosler reloading manuals.

My seating depth rules are: first, the overall length of the cartridge must fit the magazine, second, the OAL must be at least .010" from the lands to prevent jamming a bullet into the lands, and third, the OAL must allow for at least one bullet diameter to be inserted into the neck in order to provide sufficient neck tension. This has worked for me for as long as I have been reloading.

The most significant element I've found in producing accurate reloads is the selection of the bullet assuming you're using a powder that is suitable for the cartridge that you are loading for.

 

[Gargoyle]

The longer you can seat the bullet the better the case capacity for powder fill. Anymore I just find .002-.003" off the lands and tweak powder charges till I see 1/2 MOA or better.

I wonder when people say "a barrel settles in" if it really just erodes the throat till the jump is optimal.

Maybe what I said was worth a damn. LOL

 

[Ned Ludd]

I have tried for years to learn exactly what we're doing when we tune using seating depth, both from load development as well as reading the comments of others. I have read numerous posts online that suggested possible mechanisms including fine-tuning barrel harmonics, optimizing how the bullet enters the rifling, or even regulating the timing of how the case neck "seals".

As far as I can tell, it does not seem to be predominantly an issue of barrel timing/harmonics. I base this statement on numerous cases where a given bullet tunes in optimally within a specific seating depth window, regardless of velocity. In other words, loads with the same bullet at velocities of anywhere from 50-75 fps, to as much as 200 fps different all showed the exact same seating depth optima. I cannot completely rule out that by some coincidence that the bullet in each of these loads was exiting the bore at very similar spots in the barrel harmonic cycle. However, that seems pretty unlikely to me do to the wide range of velocities over which I have observed basically the exact same seating depth optimum with a given bullet.

So I keep coming back to seating depth playing a critical role in some aspect of precision that is bullet-specific, such as bullet entry into the rifling, but I really don't know with any certainty what is actually happening. That notion would certainly fit well in the context of bullet ogive profiles (i.e. tangent versus secant) and differences how those bullet profiles affect optimal seating depth windows. One thing I do know with certainty is that tuning with seating depth is not only a critical component of load development with most of the bullets I regularly use, the effect can be very dramatic. In many cases, a single .003" increment is sufficient to go from relatively mediocre precision to shooting one ragged hole. So whatever may be happening when we tune using seating depth, the effect can be very dramatic.

 

[jackieschmidt]

The longer you can seat the bullet the better the case capacity for powder fill. Anymore I just find .002-.003" off the lands and tweak powder charges till I see 1/2 MOA or better.

I wonder when people say "a barrel settles in" if it really just erodes the throat till the jump is optimal.

Maybe what I said was worth a damn. LOL

You might be on to something.

On my 6PPC barrels, I set them back on a regular basis, usually at the 300 round mark, keeping the throat fresh.
I will do this twice, then at around 700, chop the entire thread off and treat it like a new blank.

I do not do this as often with my 30BR, it simply does not wear the throat anything near what the 6PPC does.

 

[Beiruty]

I have tried for years to learn exactly what we're doing when we tune using seating depth, both from load development as well as reading the comments of others. I have read numerous posts online that suggested possible mechanisms including fine-tuning barrel harmonics, optimizing how the bullet enters the rifling, or even regulating the timing of how the case neck "seals".

As far as I can tell, it does not seem to be predominantly an issue of barrel timing/harmonics. I base this statement on numerous cases where a given bullet tunes in optimally within a specific seating depth window, regardless of velocity. In other words, loads with the same bullet at velocities of anywhere from 50-75 fps, to as much as 200 fps different all showed the exact same seating depth optima. I cannot completely rule out that by some coincidence that the bullet in each of these loads was exiting the bore at very similar spots in the barrel harmonic cycle. However, that seems pretty unlikely to me do to the wide range of velocities over which I have observed basically the exact same seating depth optimum with a given bullet.

So I keep coming back to seating depth playing a critical role in some aspect of precision that is bullet-specific, such as bullet entry into the rifling, but I really don't know with any certainty what is actually happening. That notion would certainly fit well in the context of bullet ogive profiles (i.e. tangent versus secant) and differences how those bullet profiles affect optimal seating depth windows. One thing I do know with certainty is that tuning with seating depth is not only a critical component of load development with most of the bullets I regularly use, the effect can be very dramatic. In many cases, a single .003" increment is sufficient to go from relatively mediocre precision to shooting one ragged hole. So whatever may be happening when we tune using seating depth, the effect can be very dramatic.

Custom heavy rifle setup for BR or ELR with heavy long barrel are inherently accurate and they might not respond to tuning

---

 

[Dusty Stevens]

It has to do with pressure/speed and when the bullet exits on the vibration movement

 

[NZ_Fclass]

I never found seating depth to be a significant element in developing accurate loads. I'm referring to the seating depths often reported in published reloading data for specific bullets, especially in the Sierra and Nosler reloading manuals.

My seating depth rules are: first, the overall length of the cartridge must fit the magazine, second, the OAL must be at least .010" from the lands to prevent jamming a bullet into the lands, and third, the OAL must allow for at least one bullet diameter to be inserted into the neck in order to provide sufficient neck tension. This has worked for me for as long as I have been reloading.

The most significant element I've found in producing accurate reloads is the selection of the bullet assuming you're using a powder that is suitable for the cartridge that you are loading for.

Perhaps it is the selection of bullet that adjusts your seating depth? I'm not sure why the concern of jamming the bullet - I have one rifle that performs best with a 10thou jam - but I would be wary of jamming a high pressure load. But I do appreciate that loading for magazine rifles is a different art.

 

[El Matador]

My opinion is that seating depth affects the pressure wave more than the timing of the bullet. When the powder ignites and the bullet enters the throat there is a sharp spike in pressure, which causes a wave to propagate down the barrel. When it reaches the muzzle it turns around and bounces back. It may go back and forth a few times before the bullet exits, and the theory is that you want the bullet to fly out when the chaos is traveling around near the breech end. If the pressure wave is moving a lot faster than the bullet then it would stand to reason that small changes in the shape and timing of that wave would make a difference in accuracy. Powder charge would have more of an effect on the amount of time your bullet spends in the bore.

 

[boltfluter]

.010 into the rifling is always my starting point with most calibers that I shoot. Have never pulled a bullet in any of my hunting rifles with this setting. Most times this seating depth is the one to beat! I think, when full length sizing, the bullet gets a good straight start down the bore. This is assuming you have the correct powder. Can I prove this, nope. It just flat works for me. That's all I have to offer. Happy shooting!

Paul

 

[C.E.Smith043]

Barrel time- exit point in the vibration cycle of the barrel moved by a few milliseconds to find the node of best accuracy.

Spot on. Short and to the point.

 

[Intheshop]

Lots of good insight above.... reluctant to post as most here know,I'm a cast only shooter...

It would be entirely possible to write an equation that could be used as a quick guideline.... I'm too old and wore out.

There's a cpl elements that are also in this. Some are, the leade to ogive geometry,loaded round neck/chamber clearance,and where the bullet base is in relation to case interior. JMO,but the reason a med. to hard'ish jam works for so many is that it circumvents the ogive geometry above..... I've "seen" this because we routinely change ogives through swage dies. So in effect,back to back comparisons of this delicate interface.

It's a WHOLE lot deeper,subject wise.... I am not a writer so end up sorta adding confusion instead of clearing it up. Good luck with your project.

 

[BartsBullets]

I’m talking a Benchrest rifles (short and long range) that need to shoot as small as possible. Seating depth is critical. When I leave for a match the one thing I want nailed down is the seating depth.

Bart

 

[Acyr]

My opinion is that seating depth affects the pressure wave more than the timing of the bullet. When the powder ignites and the bullet enters the throat there is a sharp spike in pressure, which causes a wave to propagate down the barrel. When it reaches the muzzle it turns around and bounces back. It may go back and forth a few times before the bullet exits, and the theory is that you want the bullet to fly out when the chaos is traveling around near the breech end. If the pressure wave is moving a lot faster than the bullet then it would stand to reason that small changes in the shape and timing of that wave would make a difference in accuracy. Powder charge would have more of an effect on the amount of time your bullet spends in the bore.

I believe this has merit.

 

[pertnear]

Interesting thread. There is little I can contribute except that my starting point is always the ogive .010 off the lands. Most of my rifles are hunting/varmint rifles. One consistency noted in the replies is that a minute change in seating depth can make a BIG difference in precision (i.e. 50% tightening an already good group). The reasons why are varied & are still speculation.

I'm not sure how shooters fine-tune seating depth either. I don't reload at the bench but have thought it would be pretty nice to have a seating die set-up while shooting. For me, once I've found my great load I have to load a series at various seating depths & head to the range. This, for sure, has me left wondering if I ever really found that sweet spot seating depth.....

 

[mikecr]

I think different than everyone here, but you're looking for all angles.
For anything but 6PPC tuning adjustments, I don't think optimum seating relates to pressure or barrel timing.

I think optimum seating, as actually determined through full seating testing, is a point of best bullet-bore interface. A place where the bullet happens to engage rifling with least rattle, delay, offset, or angle.
And as far as I can see, this passes all tests.

 

[C.E.Smith043]

I think different than everyone here, but you're looking for all angles.
For anything but 6PPC tuning adjustments, I don't think optimum seating relates to pressure or barrel timing.

I think optimum seating, as actually determined through full seating testing, is a point of best bullet-bore interface. A place where the bullet happens to engage rifling with least rattle, delay, offset, or angle.
And as far as I can see, this passes all tests.

I would have to disagree. I've seen seating depth changes make huge differences shown on the Chrono (pressure), and on paper in group size (barrel timing)

 

[dellet]

Loading sub-sonic gas guns changed or redirected my thinking of what can happen with changes in seating depth. Small manipulations of burn rate.

Generally working with a small velocity window and function is often more important than accuracy, or at least takes priority. If it won’t feed the next round, or lock back on an empty magazine, the load is basically useless.

Also consider that most/many loads are lower density and never jammed.

When a change in seated depth changes the peak and muzzle pressure enough change whether a load can cycle the action or move the bolt another 1/4” to allow it to lock on empty, it’s changing the dynamics in the barrel. It’s something you can actually see and not speculate.

Then to plug loads into Quickload, where you can attach numbers from not only peak and muzzle pressures, but exit time and percent of powder burned and velocity, it all starts making more sense.

Just some observations