Seating Depth - Thinking Outside the Box

[jds holler]

My mind goes to weird places sometimes, -- don't know if its creativity or autism.

A VERY common discussion around here is on the importance of seating depth, and I'm pretty sure that our minds generally focus on the distance of the ogive to the lands.

BUT - what if the most important factor involving seating depth, isn't ogive vs lands, but the other end of the bullet. Such as the bullet base in relation to the neck/shoulder junction? Or even base to powder, or amount of unfilled case.

Think about the incredible difference in neck tension between a bullet that doesn't reach the neck/shoulder junction versus one that seats well below that point.
Or the difference that might occur if the shoulder of a boat-tail happens to land right at that point of the case. -- .002" one direction or the other might equate to a serious change variable.

Whadaya'll think??
jd

 

[Coyotefurharvester]

I remember something about powder shouldn't be above the boat tail on a seated bullet, on a flat base bullet powder can be above the bullet base???

 

[ELR LVR]

My mind goes to weird places sometimes, -- don't know if its creativity or autism.

A VERY common discussion around here is on the importance of seating depth, and I'm pretty sure that our minds generally focus on the distance of the ogive to the lands.

BUT - what if the most important factor involving seating depth, isn't ogive vs lands, but the other end of the bullet. Such as the bullet base in relation to the neck/shoulder junction? Or even base to powder, or amount of unfilled case.

Think about the incredible difference in neck tension between a bullet that doesn't reach the neck/shoulder junction versus one that seats well below that point.
Or the difference that might occur if the shoulder of a boat-tail happens to land right at that point of the case. -- .002" one direction or the other might equate to a serious change variable.

Whadaya'll think??
jd

With regards to the bullet being in the neck/shoulder junction having any effect
Most of us eliminate the constriction of the neck shoulder junction often called the Donut
By reaming or neck turning then fireforming then neck turn again and the donut is gone.
--------------------------
At any rate, SD is relative to bullet departure timing with the barrel vibration position.
Nodes, upswings, downswings, sideswings etc
A Node (Simplified) will be where the barrel does or repeats the same thing at the same time every time
--------------------------
But BUT!!!
There is a consideration that the seating depth does also have something to do with chaning the internal volume of the cartridge which could change/tune flame or burn characteristics

-------------------------
Although I think this last item may only contribute very minimally, say 5% or even less.
Where timing the bullets exit is 95% or more of the equation within developing a Known Constant
When in all things load development related, we are trying to convert all variables into known constants.
I derive my theory based on this one fact, not feeling, not opinion, but a fact
I can change to a different powder
which will have a different burn rate
different load density
but keep my magic seating depth
And still shoot very accurately if all I do is keep the same seating depth
which leads me to believe that 1 aspect is the biggest contributor to timing the bullets departure to the barrels resonant frequency

 

[ELR LVR]

Now we could speculate further and give in to the notion that neck tension plays a major role
some believe it does
if this is true , then the bullet being seated deeper has more neck tension than a bullet seated not as deep
Then we have the fact that some cases have longer necks than others
so then we have to take those 2 things into further consideration,
then play even more with neck tension to compensate
----------------------------------------------
Instead what I like to do is remove neck tension as a variable altogether
by making my neck tension as light as possible is has no real effect on the bullet whether it is seated in .020" or .200" there is not much more force gripping and holding onto the bullet
Those that have standard, or even heavy neck tension must compensate with different sized bushings
I simplify this by instead compensate with neck turning for a certain feel of how the brass feels it grips the bullet when seating a bullet.
I could seat a bullet with about 1 pound of pressure on my ram press arm
The weight of the arm falling itself, almost seats a bullet whether it is 22BR or 284 Win.
Therefore in all my acurate cartridges, neck tension is almost all the same across the board
and All I really have to play with is 2 things
Seating Depth
Charge Weight
---------------------
Works for me anyway

 

[Bc'z]

Setting up a dummy round for 280ai 140ttsx on right, 139gr interlock in the middle.
I was skeptical, but had to show a lil faith in the fact Barnes obviously knows what thier doing.
Base of bullet is well below shoulder body junction.

 

[ELR LVR]

Setting up a dummy round for 280ai 140ttsx on right, 139gr interlock in the middle.
I was skeptical, but had to show a lil faith in the fact Barnes obviously knows what thier doing.
Base of bullet is well below shoulder body junction.
View attachment 1666365

Yeah, that don't hurt nuthin

 

[Webster]

My mind goes to weird places sometimes, -- don't know if its creativity or autism.

A VERY common discussion around here is on the importance of seating depth, and I'm pretty sure that our minds generally focus on the distance of the ogive to the lands.

BUT - what if the most important factor involving seating depth, isn't ogive vs lands, but the other end of the bullet. Such as the bullet base in relation to the neck/shoulder junction? Or even base to powder, or amount of unfilled case.

Think about the incredible difference in neck tension between a bullet that doesn't reach the neck/shoulder junction versus one that seats well below that point.
Or the difference that might occur if the shoulder of a boat-tail happens to land right at that point of the case. -- .002" one direction or the other might equate to a serious change variable.

Whadaya'll think??
jd

Don't want to strain my brain today. Doesn't changing the seating depth change all the above variables. The target will tell you what's best. Humans cannot account for all these variables.

 

[Clancy]

Don’t worry about why. Who knows… who cares? We can only speculate.

I personally don’t think it’s got a dang thing to do with where the lands are. I think it’s all exit timing. You can find little dots with the same bullet at a multitude of depths both in and out of the lands. You’ve just got to make sure you’re in the middle of one of those happy places.

 

[NZ_Fclass]

Don’t worry about why. Who knows… who cares? We can only speculate.

I personally don’t think it’s got a dang thing to do with where the lands are. I think it’s all exit timing. You can find little dots with the same bullet at a multitude of depths both in and out of the lands. You’ve just got to make sure you’re in the middle of one of those happy places.

Correct. But not all cartridges have multiple seating depth nodes.- Is it just by chance that some cartridges are considered more "accurate" than others? Take the most winning/accurate cartridges - 6PPC, 6BR, 7 SAUM. etc. They have multiple depth nodes - how many times have we heard "easy to tune" ? And I don't mean "shoots 1MoA all day long"

We also, with magazine fed rifles, have more tuning difficulty with cartridges that approach maximum magazine length. ie - I could not tune/shoot accurate 208's in a magazine fed 308 - far easier in a long free bore single shot - where I can set the optimum length and, as there is no mechanical feeding issue, neck tension. By this virtue, I keep the bullet base out of the powder, so no "creeping" of overall length due to the push of compressed powder with light neck tension Overall, it's a balancing act between powder selection/amount, neck tension and length to achieve best accuracy.

If we apply that model to a magazine fed rifle, perhaps we look at the 6.5 Creedmoor ( and it's posh cousin, 6.5x47 Lapua ) which by design - shorter case of appropriate capacity, Good neck length and good SAAMI freebore, makes for a cartridge that can be loaded for good accuracy - certainly at short, 5/600 yd - ranges - and is "easy to tune"

 

[gunsandgunsmithing]

I see a lot of posters toss around the assumption that seating further into the lands raises pressures, as being fact...but it's really not that simple. Yes, it may well change the rise time of pressures but since peak chamber pressures continue to rise and typically happen while the bullet is say 3-4 inches down the bbl, well, that pretty much dispels that. I've tested it and watched the pressure curve and rise time as well as peak pressures change with a strain gauge system, so I know it's not always the case, too.

Yes, both ends of the bullet have some effect on pressures but it's mostly rise time, not peak pressures and sometimes, deeper into the lands actually LOWERED peak pressures a bit.

Bottom line, these are tuning aspects of loading our own ammo and affecting bullet exit timing. It's incorrect to assume that jamming always raises peak pressures.

 

[dellet]

It depends. Way too many factors for trying to establish an absolute answer.

Quick example, moving the bullet in, increases load density, increases starting pressure, increases velocity. The opposite is also true.

The wild card is the relationship of the bullet or ogive to the lands and throat. At some point the pressure increase/decrease due to jump or jam over rides the pressure increase/decrease of load density. So moving the bullet can both decrease and increase pressure and velocity, without a jam. The speed at which the powder ignites, burn rate, can also change that relationship. It’s a balancing act.

Loading subsonic 300 Blackout changed my opinion of what can be manipulated with seated depth and load density, concerning pressure a velocity. When moving a bullet out, lowering start pressure and velocity, changes muzzle pressure enough that an AR will reliably cycle and lock on empty instead of short cycling and not pick up the next round, you have dramatically altered burn rate.

Taking this a step further, 9 grains of 4227 with a 220 grain bullet produces roughly 1050 fps. In an 8” barrel and provides full function. Yet the same rifle with a 125 grain bullet, requires 11.3 grains 4227 to cycle at 1070 fps. The difference in bullet weight and bearing surface dramatically changes the pressure curve.

The above mostly deals with velocity, not as much with accuracy. A big part of accuracy is getting the bullet started straight down the bore. What happens when the bullet is seated well below the neck with powder under the tail and a gap over?
Gas pressure will be equal all around the bullet, but that solid powder follows the path of least resistance. It’s a race between getting enough bullet down the bore, before the powder under it kicks the tail off center. Normally there is enough support of the bearing surface of the bullet and the case neck to keep the bullet from going sideways, but again 300 blackout has something to teach. Since it has a Weatherby type throat, many bullets bearing surface clear the case mouth before touching the throat or lands. Complete free float in absolute turbulence. Part of the reason it’s not exactly known for precision work. Keeping the brass long, and bullets with longer bearing surfaces starts cutting group size in half.

Let’s throw a wildcard an look to history and how problems were solved 150 years ago.
One of the most accurate shooting methods had the bullets loaded separately from the case. Bullets loaded from either the breech or muzzle, to a very specific distance ahead of where the case would be in the muzzle. The load really wasn’t given as much attention as the bullet seated depth into the bore. You find load data where a shooter noted the seated measurement in the 1/64ths, probably more because it was hard for the average guy to measure any finer. This in some ways would be similar to seating a bullet in an air rifle. No neck tension available.

Start and end the same way.

It depends

 

[J_idaho]

Powder to bullet base relationships are going to effect combustion based on burn rates.
Bullet to lands relationships effect harmonics.
Seems they are two completely different things that surely affect each other in some way. And with all the different combos you can come up with some more extreme than others. I’d say neck tension effects combustion until you run to light then the bullet is popping into the lands pre ignition so that could drastically effect harmonics. Just like to much neck tension can allow a much higher burn rate before bullet enters the lands.
I think you have to trust the guys that load for benchrest, full cases seated past the donut jammed or really close to lands with thin necks for a perfect seal. It equals low vibration and consistent ignition. This is why chambers are built around the bullet. Not finding a bullet that works around your chamber.

 

[gunsandgunsmithing]

It depends. Way too many factors for trying to establish an absolute answer.

Quick example, moving the bullet in, increases load density, increases starting pressure, increases velocity. The opposite is also true.

The wild card is the relationship of the bullet or ogive to the lands and throat. At some point the pressure increase/decrease due to jump or jam over rides the pressure increase/decrease of load density. So moving the bullet can both decrease and increase pressure and velocity, without a jam. The speed at which the powder ignites, burn rate, can also change that relationship. It’s a balancing act.

Loading subsonic 300 Blackout changed my opinion of what can be manipulated with seated depth and load density, concerning pressure a velocity. When moving a bullet out, lowering start pressure and velocity, changes muzzle pressure enough that an AR will reliably cycle and lock on empty instead of short cycling and not pick up the next round, you have dramatically altered burn rate.

Taking this a step further, 9 grains of 4227 with a 220 grain bullet produces roughly 1050 fps. In an 8” barrel and provides full function. Yet the same rifle with a 125 grain bullet, requires 11.3 grains 4227 to cycle at 1070 fps. The difference in bullet weight and bearing surface dramatically changes the pressure curve.

The above mostly deals with velocity, not as much with accuracy. A big part of accuracy is getting the bullet started straight down the bore. What happens when the bullet is seated well below the neck with powder under the tail and a gap over?
Gas pressure will be equal all around the bullet, but that solid powder follows the path of least resistance. It’s a race between getting enough bullet down the bore, before the powder under it kicks the tail off center. Normally there is enough support of the bearing surface of the bullet and the case neck to keep the bullet from going sideways, but again 300 blackout has something to teach. Since it has a Weatherby type throat, many bullets bearing surface clear the case mouth before touching the throat or lands. Complete free float in absolute turbulence. Part of the reason it’s not exactly known for precision work. Keeping the brass long, and bullets with longer bearing surfaces starts cutting group size in half.

Let’s throw a wildcard an look to history and how problems were solved 150 years ago.
One of the most accurate shooting methods had the bullets loaded separately from the case. Bullets loaded from either the breech or muzzle, to a very specific distance ahead of where the case would be in the muzzle. The load really wasn’t given as much attention as the bullet seated depth into the bore. You find load data where a shooter noted the seated measurement in the 1/64ths, probably more because it was hard for the average guy to measure any finer. This in some ways would be similar to seating a bullet in an air rifle.

Start and end the same way.

It depends

Good post and to your last point..I think. Compressing powder tends to slow the burn rate a tad. Kinda like one big kernel vs lots of light, fluffy kernels with lots of exposed surfaces. Kernel size and shape along with surface treatments, is the bulk of how powder makers control powder burn rates...fwiw.

 

[jds holler]

If we apply that model to a magazine fed rifle, perhaps we look at the 6.5 Creedmoor ( and it's posh cousin, 6.5x47 Lapua ) which by design - shorter case of appropriate capacity, Good neck length and good SAAMI freebore, makes for a cartridge that can be loaded for good accuracy - certainly at short, 5/600 yd - ranges - and is "easy to tune"

That addresses one of the things that's been bouncing around in my head. With a long neck and free-bore, we often don't even have the issue of forcing the shank of the bullet past the doughnut zone.
I can think of a few cartridges which have a reputation for shooting well with almost any bullet you try. Coincidentally, they frequently have a long neck that prevents most bullets from reaching that point on the case. HMMMMM.

 

[Steve Donlon]

This is a 6.5 144 gr. berger and I have .1005 jump. The bottom of the bullet is on top of the powder column with 41.5 grs of RL 16 powder. And all the bearing surface in the neck. I could not load it at .015 off like I try to do or it would be out of the neck to much. OAL is 2.855" I seated the bullet where I thought it should be without regard to seating depth. I do this all the time when .015" jump will not work as a starting point. If the bullet likes some jump then I think position is a key factory. This loads is very accurate. No ladder test required. A 147gr. Hornady at 2.855 oal is .015" off the lands. The same OAL for both bullets.

 

[jpjulian]

If bullet, chamber & magazine allow it, I seat .020"-.030" off and go with it. Done. If the bullets are too short, ie: varmints loads in a Wylde chamber, I seat to an o.a.l. that puts the base of the bearing surface at the shoulder neck junction. Done. I see no need to complicate matters further.

 

[dellet]

Good post and to your last point..I think. Compressing powder tends to slow the burn rate a tad. Kinda like one big kernel vs lots of light, fluffy kernels with lots of exposed surfaces. Kernel size and shape along with surface treatments, is the bulk of how powder makers control powder burn rates...fwiw.

I think that depends on the powder. It’s certainly something the designer keeps In mind.

Trail boss is great example, tubular powders also. They are designed for flame travel to pass through the holes. Compress them and crush the holes and it’s a completely different powder. Often pretty volatile.

Nosler load data is great example. They not only list their most accurate load, but also the load density. Patterns start to emerge.

Back to your comment on the pressure curve and time to peak. The same powder will react differently in a 22 vs 30 caliber. Bore volume can and will come Into play.

I mention 300 Blackout a lot because it’s such a strange cartridge. Loads
From 7-24 grains. Bullet weight from 80-265 grains velocities from 950-3000 fps. There is no play book.

My favorite is a 175 SMK, loaded with 1680. From 10-21 grains. 900-2100fps. MOA all the way at 200 yards. You can watch the groups change with load density. Once you start to compress, it’s a completely different powder.

I think it’s possible that you can have both under compression, a change in burn rate. But also a change in ignition rate. Ball powders are a good example, compressed they can be harder to ignite, but when they do, it’s all at once. Thats where pressure spikes come.

What I have seen is, and this is actually a pistol powder, 296/H110. Somewhere between around 98-105% density velocity flattens. This is a huge warning sign. Somewhere 103-110% you turn a flattened primer into a blown primer pocket. In a small case the spread from 95-110% might be less than a grain.

These kind of things change the rule of thumb calculations, or at least the out come. Use the wrong rule of thumb for a powder, and the rule is you lose a thumb. Or an eye.

 

[Clancy]

View attachment 1666425This is a 6.5 144 gr. berger and I have .1005 jump. The bottom of the bullet is on top of the powder column with 41.5 grs of RL 16 powder. And all the bearing surface in the neck. I could not load it at .015 off like I try to do or it would be out of the neck to much. OAL is 2.855" I seated the bullet where I thought it should be without regard to seating depth. I do this all the time when .015" jump will not work as a starting point. If the bullet likes some jump then I think position is a key factory. This loads is very accurate. No ladder test required. A 147gr. Hornady at 2.855 oal is .015" off the lands. The same OAL for both bullets.

Looks like you have plenty of room to move the bullet up to .015 off, but if you’re happy then why bother… I guess?

 

[Steve Donlon]

Looks like you have plenty of room to move the bullet up to .015 off, but if you’re happy then why bother… I guess?

usually when I'm at .015" - .020" off I"m right there in the perfect spot like with the Hornady 147gr. ELD-M bullets. When I have a lot more jump than that, I put the bullet where I would like it to be and see what happens. Most of the time with Sierra and Hornady and even some Berger bullets .015- .020" is good. That's the way my chamber is. So I try to get bullets that fit or put them where they get the best burn and most bearing surface in the neck then go from there. Especially in the 30-06 with Sierra 210 MK bullets and Berger 215 gr. target, Berger 200-20X or Hornady 208 gr ELD-M bullets. They all like .015" off. Again, my chambers are made for those bullets to fit the way that gets the best performance all around. That's the point of having a chamber made for the bullets that you want to shoot. But you are right, could just move them up if they did not shoot at .105" off.

 

[tom]

If it puts your mind at ease, I will say I've had one barrel in my life not wanting me to follow the wear. It tells me it's relationship to barrel.

Tom