Dumb question about Seating Depth

[mram10]

Here we go.
I have been trying to get a good understanding of seating depth and why it affects accuracy. It seems the reoccurring answer is "it changes pressure". If that is the case, then wouldn't changing the powder load to match that pressure work the same way?

Example: 210vld .010 off, 90gr h1000 60kpsi
210vld .060 off, 90.5gr h1000 60kpsi

The above two loads will/will not shoot the same based only on pressures? I am guessing no, so why not find seating depth first? Does each bullet have a "sweet spot" based on seating depth alone in a particular rifle?

Please answer with testing you know of or have done.

Thanks in advance guys

 

[DanSavage]

Well, seating depth will affect pressure,, it is more about does your particular barrel prefer the bullet getting a little run to get into the rifling before the pressure peaks or maybe your barrel and bullet combo would prefer being jammed and having a faster peak pressure to get the bullet going down the barrel. The bullet can actually stop in a barrel 3 times before it really takes off. A barrel and bullet choice will prefer a certain seating depth to give you the most consistent groups.

 

[Joe R]

I'm a newbie compare to many guys here so take this with a grain of salt. Some bullets like to jump and others (VLDs) like to jammed into the lands or just short of it.

Depending on the powder and distance you're shooting, some guys need the extra case capacity to get the desired velocity so jamming is a necessity for them.

Regarding pressure, I was told that loading a bullet into the lands would cause a pressure spike, but my experience was the opposite. I've found that as I extended my bullet further into the lands the velocity actually dropped because of the larger space for the powder to expand in the case. Conversely the deeper I set the bullet into the case the velocity and pressure spiked upwards.

I also noticed that there is a sweet spot where accuracy is best. For me it turned out to be at .0012 off the lands (hard jam). As I seated the bullet deeper the groups spread to a point and then started to come together again. So for me there wasn't just one optimal seating depth. Keep in mind that I am using a 30 cal. JLK 180 grain, 30" barrel with your bullet, case, primer and rifle things will likely be very different.

Kindest regards,

Joe

 

[DanSavage]

I find the same thing as Joe ,,using the book coal data moving the bullet in the case more increases pressure and moving out of the case decreased pressure until I hit the lands pressure went up a little but not much. I generally like to start my load development jammed,, then work away from the rifling and I rarely make it to the book COAL before I find that sweet spot.

 

[DeltaBravo]

Well, I just changed 6mm bullets from Hottenstein 68BTs to Barts 68BTs. The H68s liked to be 9 thou off jam; the B68s like to be 12 off. I started at touch and backed them out 3 thou at a time until I got a nice, small 3-leaf clover. Next will be minor changes to the charge in .2 grain increments. It shouldn't take more than +/- .4 grains to get it right.

Dennis

 

[searcher]

There are forces at work that most of us either do not completely understand - or cannot explain. I had thought that when one used a milder primer, for example - one that gave, say 30 f.p.s. less velocity - that merely increasing the powder load back up to the same desired velocity would create the same result. Not so, or at least in my rifles. It would seem that pressure, as measured by velocity alone is not the only factor. Seems there are factors such as duration of the pressure curve and other influences that ultimately affect the "vibe". I am one of those that does not completely understand it - but do know that simply adjusting the powder does not always get you there. I could not find the thread to refer you to but someone here had given a very descriptive explanation. Hopefully, he or she is reading this.

 

[ballison]

Joe & Dan,
When you seat a bullet against the lands, it hits immediate resistance and you get a quick pressure spike. When you jump, the bullet gets a running start, creating a lower pressure spike but higher velocity.

Bill

 

[Joe R]

FTRinPA said:

Joe & Dan,
When you seat a bullet against the lands, it hits immediate resistance and you get a quick pressure spike. When you jump, the bullet gets a running start, creating a lower pressure spike but higher velocity.

Bill

Bill that's what I had been told and believed, but my Oehler 35 refuted that completely. The pictures below show 3 charges (42.4, 42.5 and 42.6) with a CBTO (Case Base To Ogive) of 3.245 and 3.250. As you can see the pressure drops as the CBTO gets longer. And that continues for several more iterations. By the way the bottom 2 target show a flyer, well that's my fault, I'm a bad shot and need lots of practice.

mram10,
I'm going to give you a break. You are focused on the wrong things, pressure and seating depth have a role to play, but only a secondary role. The main objective is OBT (Optimal Barrel Time), that will lead you to the velocity that you need, which then will lead you to the charge that you need use. Once you have the proper charge that gives you the proper velocity and OBT, THEN you can play with seating depth. Seating depth is the last step.

To understand OBT you go to Chris Long web site and read and digest his work http://www.the-long-family.com/. Sleep on it and let it settle in your brain then you'll have a Eureka moment. If you use QuickLoad you can get there faster, but you don't have to have it.

I have found that Chris Long's formula works even though for me it was 1% slower than his estimate. I know others have found the same 1% variance. It will save you time, money and frustration.

Good luck,

Joe

 

[mram10]

That OBT is very interesting. I might be missing something, but how can I figure out my barrel time without quickload?

 

[Erik Cortina]

Joe R, when you jam the bullet it will create more pressure. Maybe you are simply in the lands and not jamming. If jamming did not increase pressure, you must have light neck tension that allowed the bullet to "soft seat".

 

[seymour fish]

GSTaylorG, Its a fascinating subject and a great thread. Everyone here is homing in on the notion, supported by lots of empirical data, that seating dept effects the quality of barrel vibration.. Art Alphin has claimed that seating depth is less critical in a long tight parallel throat ( see: Any Shot You Want). Appears some consensus is forming. Seymour

 

[Joe R]

Wow,
very interesting stuff here.

Erik Cortina said:
Joe R, when you jam the bullet it will create more pressure. Maybe you are simply in the lands and not jamming. If jamming did not increase pressure, you must have light neck tension that allowed the bullet to "soft seat".

Erik you are correct, when I jammed my bullet hard into the lands I did get a velocity jump of about 50 fps, but as soon as I backed off that even by .003 the rate of velocity drop decreased to about 5 fps for each decrease of .003 in CBTO length. That rate of decrease was constant for several iterations then the velocity started climbing again. The only reason I can think of is that the decrease in volume of the case because the bullet was taking up more space causing velocity to increase again. Maybe someone here can think of other reasons and illuminate me.

gstaylorg said:

I understand that .012" into the lands is not a "hard" jam, which likely would raise the pressure in a more significant way. However, I am also quite sure that my bullets are not soft seated. Quickload suggests adding something like 7200 psi to the start pressure for a jammed load. I'm assuming that suggestion must be for a "hard" jam of .020" or more, because my data and observations just didn't support anything like an increase in pressure of that magnitude with the 90 VLDs seated at touching plus .010"-.012".

gstaylorg,
Thanks for sharing your experience, observations and insight, they are much appreciated and valued.

I totally agree with you that the 7,200 psi recommended by QuickLoad is too high of a number that is not supported by my data. I suspect that the amount of Freebore plays a role and is another variable that should be considered and thereby making each rifle "unique".

Having said that, I believe that Chris Long has made a seminal contribution in my understanding of barrel harmonics and load development. His work combined with QuickLoad do make my whole process of load development far more focused and systematic. Maybe it's because I'm a newbie, but I try to use all the tools/crutches available to make the process easier. I'm sure more experienced shooters have a more intuitive approach and understanding, I don't, so I have to have a model that works systematically, much like a cooking recipe.

By the way, Erik Cortina also helped me a great deal in understanding that there are several ways of measuring the lands, and that is a critical baseline factor that we all talk about assuming that others are using the same method. For that I am eternally grateful to Erik.

I wish you and all of your loved ones a Merry Christmas.

Kindest regards,

Joe R.

 

[Joe R]

mram10 said:

That OBT is very interesting. I might be missing something, but how can I figure out my barrel time without quickload?

You could get some rough estimation using Chris' s table http://www.the-long-family.com/OBT%20Table.pdf and your velocity reading to back into your barrel time, but I stress the word rough. It seems to me QuickLoad is such a great tool that any serious shooter, that has the aptitude, ability, inclination and resources, would want to have.

Kindest regards,
Joe

 

[Joe Salt]

Joe I've got a buddy that throated his out, because he didn't like the bullet in the case as far as it was, can't remember how far he went, but he took the bullet forward after he throated it. The groups opened up, but by putting the bullet back to the same seating as before, the groups tightened back up. This was at 1000 yards so you will see a lot easier any changes. So Barrel Harmonics are a big part of it.

Joe Salt

 

[CharlieNC]

I have run numerous OCW/ladder trials for several combinations of caliber, bullet, and powder type and always confirmed the POI nodes were close to the OBT prediction using Quick Load. This proved to me that the OBT shock wave harmonic is more significant than the other higher frequency barrel harmonics (eg the "whipping up/down). Using this as a basis, I don't think changes in seating depth affect the barrel time enough to cause the degree of impact on group size which can often be observed. Similarly, why would there be so much difference between secant vs tangent ogive if it's just a pressure thing? So to me it seems seating depth is more of a mechanical engagement / alignment / friction type of factor as opposed to actual pressure / velocity. Also it's not the initial or max pressure which determines barrel time and velocity, but the totality (integration) of the pressure profile.

 

[Syncrowave]

Seating the bullet such that it contacts the lands when you close the bolt can help make it concentric to the chamber/bore/case. I suspect that this is why, with my guns and loads at least, I seem to find the best accuracy with loads seated this way.