Seating depth…what does it do?

[K22]

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.

For me personally, besides the safety concerns, I cannot afford to have a bullet stick in the lands when I extract a live round in the field.

Since I'm able to achieve my minimum accuracy goals with bullets seated off the lands, I see no reason for me to create a potential functionality problem by jamming bullets in the lands even if it results in a few tenths of inch tighter groups. But that's just me.

 

[CharlieNC]

A year or so ago I checked the RSI Pressure Trace site and it showed major pressure vs barrel time effect due to seating depth. Moreso than QL or measured velocity. The pressure and bullet acceleration drive barrel time. Very enlightening.

 

[Darwin]

For me personally, besides the safety concerns, I cannot afford to have a bullet stick in the lands when I extract a live round in the field.

Since I'm able to achieve my minimum accuracy goals with bullets seated off the lands, I see no reason for me to create a potential functionality problem by jamming bullets in the lands even if it results in a few tenths of inch tighter groups. But that's just me.

We have a member with wins in LRBR that doesn’t touch the rifling. The Rabbit Hole is something else.

 

[BoydAllen]

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

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Wrong!

 

[JEFFPPC]

From the context of this thread in relationship to the name of this form, ACCURATE SHOOTING, I can only say LMAO. It sounds more and more like a political form with each side sure they are correct calling the other side name. Time to sit it out.

 

[17cal Fan]

I have always been concerned with seating depth. When I find a bullet/powder combo that shows potential, I play with seating depth. .003 to .005 always seems to perform well for me. If it has to fit the magazine that usually goes out the window. Loading single for paper or Prairie Dogs it works very well.

I gave up on C.O.L. many years ago. It's a useless number. Base to Ogive is everything in my opinion, which requires the use of a comparator of some sort. I use Sinclair comparators like these available through Brownells. Simple, accurate and effective. Dependent on a seating die stem design that avoids touching the bullet tip. If you are seating a batch based on a stem that touches the bullet tip, C.O.L. and Base to Ogive will be all over the place.

 

[BoydAllen]

Relating to seating depth there is one thing that does not often get discussed. Just like there are tuning nodes for powder charge, there are also seating depth nodes. I mention this because in the past when I did not know this I would make a small change in seating depth, see less accuracy and abandon any more movement in that direction. I did not figure this out on my own, but was told about it by one of the greatest experimenters in the short range group game, Jerry Hensler. Some of you who have been in that sport for some years will know the name.

I am not sure that we will ever know for sure why seating depth has such a dramatic effect on tuning for accuracy, and I am not sure that we really need to. All we really need is to know that when I do this, that happens.

 

[Sandstorm]

Small changes produce big results for me in my application.

 

[calgarycanada]

A year or so ago I checked the RSI Pressure Trace site and it showed major pressure vs barrel time effect due to seating depth. Moreso than QL or measured velocity. The pressure and bullet acceleration drive barrel time. Very enlightening.

Was pressure higher as bullet was seated deeper?
Reason why I’m asking is that my observation has been that velocity drops as bullet is seated deeper. Not a dramatic change but clearly shows on chrono. I believed it was due to gases escaping around the bullet during the time when bullet is travelling through freebore and haven’t sealed the chamber by being fully engaged in rifling. So I believe, longer the jump, more the time gases have to leak around the bullet.
BTW this observation was also mentioned by Keith Glasscock in one of his videos.

 

[CharlieNC]

Was pressure higher as bullet was seated deeper?
Reason why I’m asking is that my observation has been that velocity drops as bullet is seated deeper. Not a dramatic change but clearly shows on chrono. I believed it was due to gases escaping around the bullet during the time when bullet is travelling through freebore and haven’t sealed the chamber by being fully engaged in rifling. So I believe, longer the jump, more the time gases have to leak around the bullet.
BTW this observation was also mentioned by Keith Glasscock in one of his videos.

I don't remember the details. But you can find the measurements on their website.

 

[Acyr]

Has anyone tested (quantified) the amplitude and frequency of the waves traveling across the barrel as it relates to seating depth changes?

 

[dmoran]

My own perspective to: "Seating Depth effects on accuracy" is:

I believe that the transitions stages from static friction into kinetic friction thresholds of motion are primarily why we see different performance levels to seating depth adjustments. These transitions induce bullet "flutter" and instability factors at different points as the bullet are in motion and engaging the bore, that all come into play. The friction resistance factors to the applied force on the bullets from the ignited powder gases to the chambered barrel steel, are sensitive to surface tension differences between various bullet shapes, sizes, weights, and jacket material, as the bullet is propelled in motion and engaging the rifled bore.

Static Friction:

• The force between two bodies in contact, which resists the initiation of sliding motion.
• Also referred to as: “Starting Friction”.

Kinetic Friction:

• Surface resistance to relative motion, as of a body sliding or rolling.
• The rubbing of the surface of one body against that of another.
• Also referred to as: “Dynamic or Sliding Friction”.

The difference between Static Friction and Kinetic Friction is: static friction occurs when the object is stationary and trying to start moving, while kinetic friction takes place when the object is moving and in motion.

<> Our control to Static/Starting Friction is done by neck surface preparation, neck tension, and the amount of case neck holding the bullet.

<> Our control to Kinetic/Sliding Friction is done by seating depth distance in relation to the lands/rifling.

Bullet deformation is an ongoing certainty. The amount of deformation is dependent on bore/groove diameters in relation to the bullet diameter, and the bullets jacket material and core structure. Balance instabilities are caused by different amounts of bullet deformation, from one bullet to the next.

These bullet deformation variances are in part steered by the amount of friction induced flutter and to the direction it influences. This flutter pushes the bullet off center, causing the bullet alignment and deformation to be slightly off center as it is engaging the bore. The deformation variances then create certain amounts of instability between bullets, which ultimately affect the point of impacts and accuracy level on the target.

Basically we need the bullet deformation to be square all the way around the bullet, as in a harmony sense to balance. And is where a "perfect" found seating depth is more optimal to do so, and is why other seating positions can effect accuracy so abruptly (primarily to vertical dispersion).


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[White Eagle]

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.

there is a lot of truth in this may not be fact but then again what is when it comes to this?

 

[CA_Shooter]

My own perspective to: "Seating Depth effects on accuracy" is:

I believe that the transitions stages from static friction into kinetic friction thresholds of motion are primarily why we see different performance levels to seating depth adjustments. These transitions induce bullet "flutter" and instability factors at different points as the bullet are in motion and engaging the bore, that all come into play. The friction resistance factors to the applied force on the bullets from the ignited powder gases to the chambered barrel steel, are sensitive to surface tension differences between various bullet shapes, sizes, weights, and jacket material, as the bullet is propelled in motion and engaging the rifled bore.

Static Friction:

• The force between two bodies in contact, which resists the initiation of sliding motion.
• Also referred to as: “Starting Friction”.

Kinetic Friction:

• Surface resistance to relative motion, as of a body sliding or rolling.
• The rubbing of the surface of one body against that of another.
• Also referred to as: “Dynamic or Sliding Friction”.

The difference between Static Friction and Kinetic Friction is: static friction occurs when the object is stationary and trying to start moving, while kinetic friction takes place when the object is moving and in motion.

<> Our control to Static/Starting Friction is done by neck surface preparation, neck tension, and the amount of case neck holding the bullet.

<> Our control to Kinetic/Sliding Friction is done by seating depth distance in relation to the lands/rifling.

Bullet deformation is an ongoing certainty. The amount of deformation is dependent on bore/groove diameters in relation to the bullet diameter, and the bullets jacket material and core structure. Balance instabilities are caused by different amounts of bullet deformation, from one bullet to the next.

These bullet deformation variances are in part steered by the amount of friction induced flutter and to the direction it influences. This flutter pushes the bullet off center, causing the bullet alignment and deformation to be slightly off center as it is engaging the bore. The deformation variances then create certain amounts of instability between bullets, which ultimately affect the point of impacts and accuracy level on the target.

Basically we need the bullet deformation to be square all the way around the bullet, as in a harmony sense to balance. And is where a "perfect" found seating depth is more optimal to do so, and is why other seating positions can effect accuracy so abruptly (primarily to vertical dispersion).

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I think see what you're saying here. Basically, for a given rifle/barrel/bullet combination, there is a seating depth that is optimal because it deforms the bullet the least.

Does this mean that the optimal seating depth for a given rifle/bullet combination will not change as the powder charge is changed for a given amount of shots on a barrel (IE: no throat erosion)

You also seem to be implying seating depth has nothing to do with barrel harmonics?

 

[K22]

there is a lot of truth in this may not be fact but then again what is when it comes to this?

All I'm saying is that this is the experience I have had. I'm not making pronouncement on seating depth.

It might and probably does have some effect on precision when you start approaching benchrest level precision, I don't know. I guess it comes down to how much time you want to spend testing every variable and how much money you have for components. For some guys, this is the part of the sport that they enjoy.

I'm precision varmint and predator hunter where for my purposes, 1/2 moa, is the level of precision I strive for. Once I hit that in load development, I'm done. Experimenting with changing seating depths is too tedious for me. Spending hours at the bench is akin to a root canal for me. 

 

[Fuj]

With bullets sorted base to ogive, I shot a target with bullets .005
off the lands and worked on powder charges. I settled on one group
that was a 1/3rd moa clover leaf. Repeated with 5 rounds to confirm.
Same charge, I loaded 4 touching. and the group size was smaller but
not by much. Bypassing the .003" into the lands, I loaded 4 at .006"
into the lands, and the group produced in the low 2's. Sent Jeff Gates
a pic of the target, and he suggested going in deeper. Now here's the
rub.....I'm using a light neck tension, and going deeper is really just
pushing the bullet back into the case, and not really deeper into the
lands. So at this point I may go much heavier on the tension and redo
the test over the winter.

So with that said.....What is the consensus on neck tensions for bench
accuracy, or is it a per gun abstract ??

 

[Yote Hunter]

I once asked James Phillips how to tune a load and he gave me the following response: He told me that powder charge can take vertical out of groups, neck tension can alter Extreme Spread and seating depth can tune horizontal out of your groups.

 

[D Coots]

With Sr Br rifle you can induce vertical and horizontal with seating depth. It can be seen on the target. Probably really shows up at Lr.

Later
Dave

 

[Dusty Stevens]

I once asked James Phillips how to tune a load and he gave me the following response: He told me that powder charge can take vertical out of groups, neck tension can alter Extreme Spread and seating depth can tune horizontal out of your groups.

Each one can affect that but can affect the others too.

 

[BoydAllen]

there is a lot of truth in this may not be fact but then again what is when it comes to this?

Every single person that I know of who is serious about getting the very best accuracy out of his loads tunes with seating depth and sees differences.