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 How to fine-tune seating depth ?
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woods
Advanced Member

USA
2132 Posts

Posted - Apr 14 2017 :  10:06:01  Show Profile Send woods a Private Message  Reply with Quote
Hey WonderMan

Since it was not clear if you meant that I needed to learn or the Quirkload users needed to learn, let me extrapolate

Hornady on their internal ballistics page:

To illustrate the effects of variations in bullet travel before the bullet enters the rifling, we'll compare a standard load with adjustments made only in the bullet's seating depth.

In a "normal" load with the bullet seated to allow about one 32nd of an inch gap (A) between the bullet and the initial contact with the rifling, pressure builds very smoothly and steadily even as the bullet takes the rifling. Pressure remains safe throughout the powder burning period (B), and the velocity obtained - 3500 fps - is "normal" for this load in this rifle.

Seating the bullet deeper to allow more travel before it takes the rifling, as in these next two illustrations, permits the bullet to get a good running start (C). Powder gases quickly have more room in which to expand without resistance, and their pressure thus never reaches the "normal" level. Nor does the velocity; with the same powder charge it only comes to 3400 fps (D).

When the bullet is seated to touch the rifling, as in the accompanying illustrations, it does not move when the pressure is low (E); and not having a good run at the rifling as did the other bullets, it takes greatly increased pressure to force it into the rifling. As the rapidly expanding gases now find less room than they should have at this time in their burning, the pressure rise under these conditions is both rapid and excessive (F). Velocity is high at 3650 fps - but at the expense of rather dangerous pressure. Many rifles deliver their best groups when bullets are seated just touching the rifling. Seating bullets thus can be done quite safely if the reloader will reduce his charge by a few grains. The lighter load will still produce the "normal" velocity without excessive pressure.


You can go to their website to see their illustrations

Some other misc charts







From Barnes (don't know if it will copy well off an Excel spreadsheet):

http://www.barnesbullets.com/resources/newsletters/september-2007-barnes-bullet-n/

Jessica Brooks, Manager
Clay Eshom, Supervisor

“The Effect of Distance From the Lands in Relation to Pressure and Accuracy”

During a very hot day in late July, we were in the lab considering how pressure and the distance bullets were seated off the lands would change accuracy. While we knew the closer to the lands a bullet is seated affects pressure, we were curious as to what specific effects seating depth would have on accuracy in a laboratory test situation.

We decided to set up an experiment to determine what, if any, changes in pressure and accuracy were noted when moving the bullet back from the lands. For our test caliber, we decided on the .308 since there are more .308 caliber firearms than any other. We chose a 180-grain TSX and used pressure barrels chambered to SAAMI pressure barrel specifications for the test.

Once our cartridge selection was made we worked up loads which were below SAAMI maximum pressure, but within a few thousand pounds of the maximum pressure to account for variation and drop-off within a given sample. We set our first distance at 0.025” off the lands, then continued to move the distance out at 0.025” intervals for five separate groups. Our one exception was the .300 Weatherby Magnum. We started at SAAMI maximum length for the cartridge and moved back in 0.050” increments.

We divided the powders into two groups, ball and extruded, for which we worked up loads and shot groups. We thought there could be a significant difference in how the different types of powders reacted. We used the same primers for both powders — if magnum primers were used for one powder type they were also used for the other. We used Federal GM215M for the magnum cartridges and Federal GM210M for the others.

To begin our experiment, we established some basic parameters in which we would operate. These parameters included case length, distance from the lands, powder type, primer type, and what information we would record from the shooting tests. We decided at the onset to perform two tests within the experimental parameters. The first test would utilize ball powders only, while in the second test we would use extruded powders. All other parts of the test remained the same for both data collections. The one other exception was with the Weatherby data. We did not start at 0.025” off the lands since Weatherby rifles have very long throats. For the Weatherby rifle, we began at SAAMI maximum length and worked back in increments of 0.050” until we had fired five separate groups.

Cartridge loads were worked up by one of our diligent Ballistic Technicians (Shooter is his nickname) until they reached an approximate pressure of 60,000 to 65,000 psi, dependent upon the SAAMI map. The barrels were first calibrated using SAAMI calibration ammunition to ensure the pressure readings were accurate. Then the five different groups were shot in order from closest to the lands to farthest from the lands. After each group a break was taken so the barrel could cool off.

We decided to use well-known cartridges featuring a variety of case capacities. We felt this would allow for a wider spectrum of differences not only within the cartridge, but also between cartridges.

PRESSURE CHANGE

The pressure seems to fluctuate between highs and lows as distance from the lands becomes larger. For example, as the distance moved to 0.100,” pressure increased with some of the cartridges, particularly the .300 WSM and the .30-06 Springfield. Some of the cartridges had pressure increases with either the ball or extruded powders but not both. This was true with the .300 RUM and the .300 Win. Mag.

As can be seen in the chart below, pressure change was not a constant for this experiment and results were highly surprising for several of the cartridges. Keep in mind, this was not an average of tests, but the result of a single test per cartridge.

We tried to work up loads that would be within the 96% range of the SAAMI maximum pressure rating for each cartridge. This proved to be somewhat tenuous, as the pressures would drop off differently for each cartridge.

*Note: beginning on the left with shot No. 1, distance is closest to the lands, and ends with shot No. 5 on the right, which was seated at the greatest distance from the lands. This applies to all charts.



Figure 1: Pressure Change Versus Changing Distance Off The Lands

















VELOCITY CHANGE

When we began this experiment, we expected velocity to drop off as the pressure decreased. This seemed a valid, objective view following a law of physics that says forcing an object in one direction results in an equal force being exerted in the opposite direction.

Notice that velocity for both powders fluctuates, almost mirroring the pressure curves seen in the above chart. If pressure increases, velocity usually follows suit. Thus if the pressure drops, so does the velocity. However, this was not always seen in our tests because of the randomness of primer ignition and powder burning rate. We note here that when testing bullets for our regular production, we may fire a thousand rounds to ensure the bullet performs optimally, and this effect of reduced pressure and higher velocity has been seen quite often during these tests.

As the distance increases and the pressure drops, velocity drop may be minimal or non-existent. The randomness of primer ignition and powder burning rates is one of the few things we cannot account for. While shooting enough test groups to determine the overall average would be interesting to try, the average reloader would not be able to duplicate our tests.

As seen with the .300 Weatherby Magnum, the pressure drops from 64,900 psi to 63,100 psi — a change of 1,800 psi. Yet the velocity drops from 3128 fps to 3114 fps, a change of only 14 fps. Another example of this is the 308 Winchester loaded with the ball powder. Here the pressure drops from a high of 64,800 psi to 62,700 psi, a change of 2,100 psi. The velocity drops from 2639 fps to 2637 fps, or only 2 fps.



Figure 2: Velocity Change Versus To Change Of Distance Off The Lands











ACCURACY CHANGE




We wanted to find out how accuracy would be affected by changing the distance off the lands. For optimum accuracy, our standard is normally 0.050” off the lands for the best accuracy. The test did show how each rifled barrel reacted to the change.

As we documented our accuracy, it became apparent that when bullets were seated close to the lands, accuracy was good. With some cartridges, we noticed that accuracy would fluctuate as we moved the bullet away from the lands. As the bullets were moved farther from the lands, some cartridges became less accurate. Surprisingly, as the bullets were moved even farther away, the accuracy would improve again.



Figure 3: Accuracy Change Versus Change of Distance Off The Lands
















In the overall perspective, we can say that generally pressure will decrease as the bullet is moved farther from the lands. Generally, we can say that velocity will decrease as pressure decreases the farther the bullet is seated from the lands. And generally, we can say that accuracy is depending on bullet placement from the lands dependant on the rifle the bullet is fired from.

In comparing overall accuracy between ball and extruded powders, the table below lists the average accuracy for the five groups over the full spectrum of distances the bullets were seated off the lands. As seen by the results, the differences between the powders are negligible.



If you read Mule Deer (John Barsness?) over at 24hr then he has said many times to expect the velocity/pressure to decrease when seating deeper.


When someone shares something of value with you and you benefit from it, you have a moral obligation to share it with others.

After Tuesday, even the calendar says WTF!


Edited by - woods on Apr 14 2017 10:07:48
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WonderMan4
Advanced Member

USA
2846 Posts

Posted - Apr 14 2017 :  17:39:10  Show Profile Send WonderMan4 a Private Message  Reply with Quote
woodsy: just so I am clear....I am advocate in your corner prolly 90% of the time including this time.

With this issue, I have studied the effects from many sources as well as experimenting (maybe foolishly) on my own...probably foolishly by today's standards.

I have a gut feeling that you have too. Just so you know, and I ain't admitting anything, sometimes I used to shoot with safety glasses and a face shield. That was me. Would I encourage anyone to try some of the things that I did? Hell no, not even on your best day. I will not even acknowledge them on a public forum. The reason was that I had to see for myself. You could say that luck was with me and the good Lord above wasn't ready for me yet.

As you know, and everyone that has read your posts knows, you are very good as putting everything together in words and with graphics.

Don't let anyone discourage you from that.

Now if Shastaboat is wondering, I ain't kissin' your ass, just stating facts. Facts are what we are all about, not rumors of "he said, she said". The internet is full of self proclaimed experts, you just gotta be smart enough to weed out the bull**** from the real ****.
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woods
Advanced Member

USA
2132 Posts

Posted - Apr 14 2017 :  20:30:04  Show Profile Send woods a Private Message  Reply with Quote
Cool, thought so but guess I'm a little sensitive since I have been through this Berger/Quickload stuff before

Yeah, like the song says

How am I ever gonna get to be old and wise
If I ain't ever young and crazy?


https://www.youtube.com/watch?v=mpc3ffTLm_g

done a few things I won't talk about too. You reckon Shasta has?

When someone shares something of value with you and you benefit from it, you have a moral obligation to share it with others.

After Tuesday, even the calendar says WTF!

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Shastaboat
Advanced Member



USA
9122 Posts

Posted - Apr 14 2017 :  20:46:38  Show Profile Send Shastaboat a Private Message  Reply with Quote
quote:
Originally posted by herbeapuce

thank you very much for you help guys.
if the gun is a keeper, I will consider this conversion kit.
I need to shoot it more, and right now I don't have much time...
stef.



You will never reach the potential of your rifle with the slow/long striker fall and military trigger.
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Shastaboat
Advanced Member



USA
9122 Posts

Posted - Apr 14 2017 :  20:50:33  Show Profile Send Shastaboat a Private Message  Reply with Quote
Excellent posting Woods.
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