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Author Topic: Seating Depth Fact & Factors  (Read 1110 times)

Offline Dutch-Hunter

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Seating Depth Fact & Factors
« on: April 24, 2014, 06:07:41 PM »
I thought we should spend a little time discussing seating depth. Some handloaders and shooters alike are unaware of the intense effects that bullet seating depth can have on the pressure and velocity generated by a rifle cartridge. This dramatically affects the accuracy of your loads. Seating depth is one variable that can be used to fine tune accuracy. SAAMI provides industry standards for guns and the ammunition used in them. Some of the specifications are max pressure and cartridge dimensions. The only bullet dimension SAAMI stipulates is the diameter.  These standards are used by rifle makers, ammo makers and handloaders so their products to all work together. With the components available today some of these SAAMI standards are in many cases outdated and can dramatically restrict the performance potential of a firearm and its ammunition.

Effects of Seating Depth on Pressure and Velocity:
The primary advantage of loading a cartridge long is increased internal volume of the cartridge. The primary adverse effect of loading long is magazine feeding. The extra internal volume created by loading long or shallow seating for a given powder charge will generate less pressure and lower velocity because of the extra void space. Another way to look at this is you have to use more powder to achieve the same pressure and velocity when the bullet is seated out long. In fact, the additional powder you can add to a cartridge with the bullet seated long will allow you to achieve greater velocity at the same pressure than a cartridge with a bullet seated short.

When you think about it, this makes perfect sense. After all, when you seat the bullet out longer and leave more internal case volume for powder, you’re effectively making the cartridge into a bigger cartridge by increasing the size of the combustion chamber.

Geometry of a Chamber Throat and Magazine
The chamber in a rifle will have a certain throat length which will dictate how long a bullet can be loaded. The throat is the forward portion of the chamber that has no rifling. The portion of the bullet’s bearing surface that projects out of the case occupies the throat. Each and every chamber is dimensionally unique. Determining this dimension has become easier with today’s tooling.

Geometry of the Bullet
The spitzer bullet can be modeled geometrically in three pieces:
1.Ogive (Nose): The ogive shape forms the front of the bullet. The ogive shape is formed from the arcs of two      circles. The ogive is what opens the pathway of flight.
2.Cylinder (Bearing Surface): The cylindrical portion of the bullet is what engages the rifling of the barrel. The cylinder is what gives stability to the bullet in flight. This stability factor is mathematically determined.
3.Frustum of a Cone: The back of the bullet (aka “boattail”) geometrically is in the shape of the frustum of the cone Tapering the back of the bullet reduces drag, particularly at speeds less than supersonic.

Only the Ogive and Cylinder are relevant to this article. The geometric ogive formula each and every bullet maker uses can be proprietary and different. These differences may be small, but they are far significant in precise loading. The nose can fluctuate by as much as .025 between bullet makers and even in the same lot.
All noses are principally a curve that is part of a larger circle. You would think this would make nose shapes fairly consistent. The problem is that the circular arc geometries are different for each bullet design. Even for a given bullet making tool is not a precise enough process to replicate these shapes exactly the same from tool to another. Add to this the challenge of putting this curve on a surface that is round. Doing this means that the size and location of the curve is influenced by the diameter of the bullet.

Cartridge Over All Length (COAL) vs Cartridge Base To Ogive (CBTO) Methods

Using COAL method:
Chamber dimensions must adhere to SAAMI specifications and tolerances; however each one is uniquely different do to these very tolerances. The length of the throat determines how much of the bullet can stick out of the case if the magazine allows. When a cartridge is chambered and the bullet encounters the beginning of the rifling, it’s met with hard resistance. This SAAMI COAL dimension is the maximum length cartridge. When a bullet is seated out to contact the lands (jammed), its initial forward motion during ignition is immediately resisted by a grooving force of the lands. Seating a bullet against the rifling causes pressures to be elevated noticeably higher than if the bullet were seated just a few thousandths of an inch off the rifling (jumped) allowing momentum to starting the engraving of the bearing surface.

A very common practice in reloading is to establish the COAL for a bullet that’s seated to touch the rifling. This is a reference length that the handloader works from when searching for the optimal seating depth for precision. Many times the best seating depth is with the bullet touching or very near the rifling. However in some rifles, the best seating depth may be 0.100” or more jump. This is simply a variable the handloader uses to tune the precision of a rifle. The longer jump requires precise runout tolerances. Strict attention needs to be paid to keeping the bullet perfectly parallel and centered in the throat. This is necessary to keep the bullets center of gravity true.
When a handloader is working to establish a seating depth to use with a particular bullet, he must decide if he needs the cartridges to feed through a magazine or not. If the shooting application is hunting or tactical shooting, then the shooter will need the rounds to cycle through the magazine so the rifle can be used as a repeater. In single fire target shooting applications, it’s not.
Relying on COAL is not exact enough for precise accuracy. The largest factor that makes this an inaccurate dimension is the bullet nose variance discussed above.
Using the CBTO method:
CBTO is a more exact way to measure seating depth for precisely loading ammunition. Due to the complex dynamics of internal ballistics which happens in an instant, the distance a bullet moves out of the case before it engages the riflings is highly critical to precision potential. Therefore, in order to analytically optimize the precision of handloads, it’s important that the precision handloader recognizes how to alter bullet seating depth in relation to the riflings.
The challenge is learning how to precisely and repeatedly measure the CBTO dimension. The diversity of bullet nose shapes makes this measurement difficult and yet critical. When your bullet seater touches the tip of one bullet, the distance to the point on the nose that transitions to the cylinder and engages the rifling is fixed. If your bullets have precisely the same nose curve and the same diameter, then your CBTO will be very uniform and should easily be able to maintain a tight tolerance. This is achieved when using same lot bullets and a seater die that does not allow the bullet to bottom out (within the seater die cone) on the tip of the bullet.

Problems of Measuring, Recording and Communicating CBTO
There is a lack of uniformity in comparators and measuring devices used to determine CBTO. This is a critical point to understand. To measure from the base of the cartridge to where the bearing surface begins on the bullet, you must use a gauge that attaches to your calipers and which also goes over the nose of the bullet to touch the point where the bearing surface transitions into the nose curve. Bullets can and will vary in this area from type to type and lot to lot. This makes it impossible for gauge manufacturers to use one given diameter and shape in their gauges. So there is no standard shape for diameter for gauges. Gauges can and will vary in both inside diameter and the shape where the gauge contacts the bullet. There is another reason why these gauges are not standardized. Since bullet nose shapes and diameters will vary, gauge manufacturers know that gauge standardization is impossible.
The common way (although not perfect) to get this dimension is to use the Hornady Overall Length Gauge and Comparator Set. This device allows you to push a modified case into a chamber that holds a bullet in the neck loosely. After the case is inside the chamber, you push the bullet forward with a rod until it stops at the rifling. You then tighten a screw into the rod to set the dimension. You remove the device and use either a cleaning rod, wooden dowel or small brass rod to put into your muzzle to knock the bullet out of the rifling (since there is no neck tension to pull the bullet out). After you get the bullet out, you put it in the top of the case with the rod fixed into the locked position. This also gives you a representation of the distance from your bolt face to where the rifling begins. The next step is to use the correct caliber comparator bushing for the caliber and your calipers to measure from the vase of the case to the ogive. However (here’s where it’s not perfect), this method is not totally accurate because the modified case you use is not fire formed to precisely match your chamber. This means that the end of the case is not likely in the precise location as your bolt face. You can get one of your fired cases fitted to go in the device but this is only good for that barrel/chamber.
Benefits of having a uniform CBTO
There is another aspect to knowing your CBTO when checking your COAL as it pertains to performance. With good bullets, tooling and carefully prepared cases, you can easily achieve a CBTO that varies less than +/- .001”; but your COAL can vary as much as .025. This is not necessarily bad and it is much better than the other way around. Something to watch out for when using this method is having the COAL coming out uniform and the CBTO varying. Example: If you have a CBTO dimension that varies while the COAL dimension is tight (within +/- .002), then it is most likely caused by the bullet is bottoming out on the bullet tip inside the seater cone of your die. This is rare but must be avoided. About the only fix is a new seating die or possibly a new seater cone for your die.

In summary; if you load using COAL you may indeed be seating your bullets with a varying seating depth. The reason for concern is inconsistent internal ballistics caused by the varying nose to bearing surface and irregular seating depths. This variance ultimately gives you different pressures and velocities and makes precise loading unattainable. However, if you achieve a level of accuracy you’re happy with then by all means stay with it. If you chose to use the CBTO method you can uniformly control the internal ballistics and carry a higher degree of precision to your cartridges. The sacrifice may be cartridges that may not magazine feed. The cost of a length gauge and comparator set is around fifty dollars and is an inexpensive investment in the pursuit of accuracy.

Happy Shooting... Dutch
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Offline Gutshot

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Re: Seating Depth Fact & Factors
« Reply #1 on: April 24, 2014, 10:07:40 PM »
Lots of good info here, I had to read it several times for this pea brain to comprehend. Thank goodness for printers.  @--0--0123
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Offline BoBallistic

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Re: Seating Depth Fact & Factors
« Reply #2 on: May 17, 2014, 03:46:25 PM »
Dutch, What planet were you born on? We all heard that Albert Einstein had a much younger brother! Are you him? You sort of look like him....LOL....

Another good, no great article...this one should go in your book too! You may want to post these articles again in September or October.....

I agree with you that SAAMI dimensions are outdated, but it is a starting point. I am a big single shot person. I have 5 Ruger #1's and a few Coopers that are single shot and a few others. Have turned a few repeaters into single shot rifles because of their seating depth require for the best accuracy out of that particular rifle for a particular bullet...Rifles 20 years ago used to base the magazine length on SAAMI lengths and there was no room for different COL of your reloads. Today's manufacturers have learned and have made their magazines longer to accommodate the longer Ogive pills. But when they give us the extra length, we have to give up a little stiffness in the stock that may affect accuracy and surely affects harmonics.

Seating depth is a variable from rifle to rifle that ever reloader need to experiment with. SAAMI length's listed in every reloading manual is just a starting point. Amazingly, only a few thousands of an inch can dramatically change the pressure inside the chamber and the accuracy down range. In my experience, most rifles, not all, when the COL is a little longer than SAAMI standards call for. I have some rifles that work best if they are .010" off the lans and some work best if they are .050" off the lans! 

I am also a big fan of flat base bullets for hunting. To me the pressure on the frustum can change depending on the amount of load density of your load and seating depth. A flat base bullet will have even pressures across its surfaces than frustum bullet do. That even pressure will effect the bullets journey down the barrel. I think it cuts down on the yaw of the pill before it hits the lans. But if you are a long range shooter, then you do want a boattail bullet for sure....For the distances I shoot (300 yards and under), the old Sierra ProHunter with it flat base works very well for me but so does the Bergers, Hornady's and Barnes who all have boat tail designs, go figure....LOL...

Some of the Bench Rest shooter I know do seat the bullets out so they do touch the lans and they cannot eject them as ammo, the pill will separate from the case if they try to remove it. Remember how we use to unload our old muzzle loaders but shooting them in the ground? That is how some bench rest guys unload their rifles by shooting that round into a dirt bank.

You have hit the nail on the head about having problems measuring, recording and communicating CTBO! We all are seeking a tool, any tools that will help us! I have both of those tools for most of my calibers and they work great! More tools you have, the more time it takes but as you said, the benefits are worth the time involved into your load.

MidwayUSA, Cabelas and Bass Pro shops are good for the average person, but if you want precision tools and toys, you need to look at Sinclair International and Brownells websites and a few others. Have been getting most of my precision tools from them for years now.

Another great article by Dutch. Will this also go into your book? I can see it now on the best sellers list! The Dutch Standard....catchy?

Thanks again!
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Offline gobihawk

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Re: Seating Depth Fact & Factors
« Reply #3 on: May 19, 2014, 04:48:05 PM »
Great info as always. Love that Hornady OAL Gage. Reloaded for years without it. What a great time saver. I was thinking the same thing Bo. Dutch should write a best selling book. I was thinking more like "Go Dutch or Stay Home".   @--0--0118 
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