Seating Depth - Thinking Outside the Box

[Fast14riot]

I typically only measure distance to lands once, in a new barrel to establish my limit. After that, all measurements are only relative to each other, for the life of the barrel. Seating depth is a function of the loaded round, not a relationship to the barrel. I can usually change lots of bullets and if my seating depth of my cartridge is the same, it's normally right in line with my expected dispersion.

 

[Doom]

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???

Actually I believe the issue is the bullet shouldn't extend into the shoulder as this changes the geometry of the powder column and and result in inconsistencies in the powder burn. Makes sense but the the degree to which it matters obviously depends on case fill and burn rate.

 

[ELR LVR]

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.

Nailed it!
Except for one thing, anytime I've seated a bullet just kissing the lands, or right at touch
Goofy unpredictable things always happen
Seat a bullet anywhere but just at touching.
Even taking the bullet back .005" off the lands alleviates this issue of unpredictability

 

[SeabeeKen]

Doesn't primer impulse move the bullet and powder forward changing all this?

Thread 'Primer impulse and bullet movement'

Aug 17, 2022

Came across this Army Research Lab report.

• SeabeeKen
• Replies: 11
• Forum: Reloading Forum (All Calibers)

• 

 

[K22]

What do I think?

1. Seating can be a factor in precision shooting. The only way to know is to test it. I have rifle / bullet combo that like a long jump and others not so much. With Remington 700's, you have no choice with most bullets, a long jump is the only option because of the large amount of free bore yet my rifles shoot just fine.

2. I do believe there can be something related to case fill because seating bullets deeper produces less air gap. I have one rifle that seems to bear this out. By reducing seating deep by 0.020", I reduced dispersion from 0.90 to .048" (five shot groups).

3. I never seat bullet closer than about 0.010" off the lands and most of the time at least 0.020" because functional reliability and safety is important to me and I do not want to get a bullet stuck in the lands or have a pressure surge.

However, while I consider myself a precision shooter (as opposed to an indiscriminate blaster), I am not a benchrest or competitive shooter.

 

[divingin]

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???

Possible if you seat deep enough, isn't it? The flat base, I mean.

 

[ELR LVR]

Doesn't primer impulse move the bullet and powder forward changing all this?

Thread 'Primer impulse and bullet movement'

Aug 17, 2022

Came across this Army Research Lab report.

• SeabeeKen
• Replies: 11
• Forum: Reloading Forum (All Calibers)

• 

Pg 16 of that report states
"(i.e., the propellant bed acts as an incompressible fluid when initially acted upon by the primer output)."

So it makes sense the bullet moves as soon as the primer is set off.
Great report to read since there was a discussion recentoy regarding this and when the bullet actually begins to move or engage the rifling
Upon primer initiation
or
After Enough pressure has been built up by the primer igniting the powder and the powder beginning to burn.
----------------------------------
The position Where we START the bullet from though, still plays a role within the ramp up of the pressure curve relative to its exit timing.
Or--- it would be incorrect to think that
1. The primer goes off
2. the bullet hits the rifling and then completely stops
3. pressure has t09 build up again for it to start moving
4. then it accelerates on out the bore
-------------------------------------
When instead
1. The primer going off gets the bullet going due to the powder column pushing against its base....
2......it may slow down as it engages the rifling, but pressure is rapidly building and even though it may slow down as soon as it hits the rifling some, it is still continually moving.
3. Therefore, where we START the bullet off from, is still important since the exit timing will change with respect to its initial seating depth.
-------------------------------------
Otherwise, seating depth would be completely irrelevent and any bullet exit timing "TUNING"
would be solely dependent upon freebore distance
and as we all can prove, all things staying the same.....when changing seating depth alone, shows changes in bullet POI and shot to shot dispersion (Aka: accuracy)
-------------------------------------
Conversely: I can take a really good tight shooting load, and turn it into a bad shooting 1 MOA+ load
just by changing seating depth alone.

 

[dellet]

There is no correct or end all answer. There will always exceptions to any rule.

Anything you do that changes the pressure curve, it changes the exit timing of the bullet. It’s absolutely possible to have the exact same velocity with different exit times, and of course the opposite is true. You can have the same exit time with different velocities.

This is why no matter how much science and theory you apply, about all it does is cut down on range time, components used and allow more shots in actual use instead of wearing out a barrel developing a load. It does not really replace good old trail and error and appease the ultimate judge, the target.

The biggest problem is sometimes applying what works in one cartridge to another. You use the same bag of tricks, just different ones get pulled out. Might even happen by merely changing a bullet.

At some point it all boils down to what happens down range. So far pressing keys, still yields to pressing a trigger for actual results.

 

[White Eagle]

I was told at one point that when a bullet base touches the powder column that is when you get the best accuracy
this was in reference to straight walled handgun cartridges like the 41 magnum

 

[Bronsin]

It would seem to make logical mechanical sense that a bullet with 3 times as much bearing surface in the neck would require less neck tension to hold the bullet. I tested exactly that, and from the results I got, I don't put any stock in the idea that how tight or loose the neck holds the bullet has any effect on accuracy in itself. The wrong amount didn't work with either bullet, and the right one did, despite the big difference in bearing surface.

I believe that neck tension affects the consistency of the ignition of the powder. When it was too light, it didn't shoot jammed or jumped, even though the SD's were better with jam. With the right tension, the SD's were the same jammed or jumped, the rifles shot their best groups, and the accuracy window was much wider.

Taking the time to do a proper NT test is well worth the effort. As to the stuff of changing the volume of the case changing with seating depth affecting pressures? That's just stuff to theorize on and argue about. I completely ignore it. We better start weighing the volume of our cases if that's got anything to do with it.

 

[Steve Donlon]

Nailed it!
Except for one thing, anytime I've seated a bullet just kissing the lands, or right at touch
Goofy unpredictable things always happen
Seat a bullet anywhere but just at touching.
Even taking the bullet back .005" off the lands alleviates this issue of unpredictability

I remember when the Sierra TMK bullets came out and I tried my starting point of .015" off with no luck with the 30 cal. 195 gr. TMK bullets. Then I read here that the golden spot was .005" off and that made all the difference. Your statement just reminded me of the .005" off and it seemed a different amount of jump compared to all the rest of the Sierra bullets but it was the spot. Got to keep that in mind, thanks

 

[357Mag]

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

Dellet -

Howdy !

Hey…. how ‘ bout shooting with NO neck… on a shouldered case ?

Pic shows “ Dead Center “ duplex .350” cal swayed lead polymer tipped bullet
w/ rebated boat tail…. wrapped w/ DYMO nylon self-stick label maker tape.
Bullet weighs 195 gr. On Lt side of pic….bullet shown sitting in a .35 Remington case, BUT, in this configuration; neck OD will not allow chambering of the case.

Same pic on its RT side, shows my wildcat “ .35Remington Neckless “, w/ the wrapped bullet sitting where it interfaces w/ the flat Fwd face of the case mouth. That interface provides the burning powder’s gas sealing mechanism… where bullet is seated w/ a
“ jam “.

2nd pic shows homemade breech seater tool, fashioned from a length of plastic drinking straw afixed to a 90* Torx wrench. Plastic straw section can slide on the Torx wrench, for adjustment.

3rd pic shows Lt side view of my M-336XLR .35Remington on the bench.

Combo easily provided “ minute of deer “ accuracy, and beyond.

“ Poor man’s Schuetzen rifle “


With regards,
357Mag

 

[ORjeff]

I suspect that what's going on to affect accuracy with changes in jump is that more jump means the bullet hits the lands with more velocity. That affects how the bullet deforms as it enters the rifling and also changes the amplitude of the vibrational energy created in the barrel (how hard the hammer hits the bell). Just my thinking about this.

BTW...the proper way to calculate jump is the difference between the chamber shoulder to the lands distance and the cartridge shoulder to ogive distance.

 

[Homerange]

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

There are so so many variables....

Mag length for us hunters dictates what we can do and I had this rammed home when developing a 222 load where with the same charge behind my preferred 223 50gr Nosler BT went overpressure yet a 50gr Hornady #2240 was just fine and shot very well (so much better !) and didn't upset case/load volumetric efficiency like the boat tail projectile did.

We see similar things with 30.06 vs 35 Whelen with the same projectile weights where the Whelen will outperform the mighty 30.06 with heavier projectiles any day.

There is no definitive answer for all calibers.....

 

[urbanrifleman]

Actually I believe the issue is the bullet shouldn't extend into the shoulder as this changes the geometry of the powder column and and result in inconsistencies in the powder burn. Makes sense but the the degree to which it matters obviously depends on case fill and burn rate.

The result is that the bullet is seated into the funnel shaped portion of the neck and after a few reloads becomes a donut. The seating pressures go haywire and it starts shooting like a shotgun.

 

[6.5 forever]

The result is that the bullet is seated into the funnel shaped portion of the neck and after a few reloads becomes a donut. The seating pressures go haywire and it starts shooting like a shotgun.

Awhile back I started seating in front of the doughnut and accuracy improved. I am now using the same seating depth on new cases. Also my velocity picked up 8-10 fps. I don't know why, but it works.

 

[AlNyhus]

If you have donuts in the neck and/or they are funnel shaped, the fix is easy. No need to struggle with something so easy to address. -Al

 

[Coyotefurharvester]

If your not necking up brass how does a donut form? With my 7saum hunting rifle(and my 30TC) bullet base is usually down below the neck well into shoulder, unavoidable with heavies or solid copper bullets in mags.

 

[Clancy]

If you’re not necking up brass how does a donut form? With my 7saum hunting rifle(and my 30TC) bullet base is usually down below the neck well into shoulder, unavoidable with heavies or solid copper bullets in mags.

The neck/shoulder junction moves forward when the case is fired. It moves backward when the case is sized. That “accordion”motion causes some buildup between.

I’d build the next one on a medium or long action, whichever is needed to suit your bullet of choice.

 

[gunsandgunsmithing]

If your not necking up brass how does a donut form? With my 7saum hunting rifle(and my 30TC) bullet base is usually down below the neck well into shoulder, unavoidable with heavies or solid copper bullets in mags.

Your logic is somewhat right but there is also some amount of brass flow with multiple firing/sizing ops that ends up at the neck/shoulder junction, creating a bit of a donut. But yeah, necking up creates a healthy donut due to the brass that becomes the lower portion of the neck came from the shoulder, which is typically thicker. Bottom line, while not often severe, slight donuts can occur without necking up.