AFRICA HUNTER QUEST©   

    Chapter 26 - THE PILGRIM DISCOVERS THE ESSENTIALS OF SEATING DEPTH AND EXTRACTOR GROOVE MEASUREMENTS   

    Donny got his gun case, placed it on the table, and opened it to reveal his rifle with the bolt in a closed position.   

    GG:    I would suggest you get a bolt holster to store and protect the bolt and keep it out of the action. How you now store the bolt could indicate the rifle is potentially loaded. Either that, or keep the bolt open in the action. It demonstrates an emphatic safety statement that makes somewhat paranoid geezers like me feel better.   

    D:    Good point.   

    GG:    Take the bolt all the way out and put your rifle in the gun vise that’s clamped to the table. Then you will need to remove your suppressor so that the wooden dowel can fully fit inside the barrel. You will need your plunger-style bullet seating depth tool with your tapped, fire-formed 270 Winchester case, your electronic caliper with the anvil, a bullet comparator, and the ¼ inch dowel. We are going to determine the seating depth needed for the testing. I will show you how it’s done. This determination is fundamentally important for achieving a truly accurate load. If I have cleaned the barrel, I check the seating depth with this method prior to loading any more rounds.   

    GG threaded the fire-formed 270 Winchester case onto the exterior sleeve of the device. He took one of the loose, upside-down bullets from Donny’s ammo box and slipped it into the case neck, then adjusted the piston inside the sleeve so that it positioned the bullet with its ogive protruding slightly above the case mouth. He screwed down the cinch screw so that the piston held the bullet in place, then inserted the tool with the affixed case and bullet into the rifle’s chamber.    

    GG took several seconds to satisfy himself that the case was pushed in all the way with the case shoulder firmly seated against the chamber shoulder, then loosened the cinch screw on the internal piston. He gently moved the piston forward, and in doing so, moved the bullet forward into the chamber’s throat. When the piston stopped, indicating that the bullet’s ogive was in direct contact with the barrel’s lands, he gently cinched back the screw to hold the piston’s position. He then removed the tool from the rifle’s chamber, and inserted the wooden dowel into the muzzle end of

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the barrel. He used the dowel to poke the bullet out of the chamber. He retrieved the bullet and inserted it back into the tool, then held the entire tool up and showed it to Donny.   

    GG:    What you are looking at is how a loaded round would look if the bullet were seated so that its ogive just touched the lands. This is a skill dependent on technique that requires repetition to perfect and get repeatable results. I am ham-handed, so I do this until I get the same measurement at least three times. The reason I do this is all my loads are referenced by either how far out or how far into the lands I seat the bullet, based on this measurement. I will tell you right now that a variation of 0.001 inch can make a huge difference in accuracy. If you are after uber accuracy, this measurement is critical.   

    “Uber accuracy,” thought Donny. “The Old Man obviously doesn’t believe in anything else. There’s absolutely no arguing with his proof targets. I suppose he figures if he is gonna be a bear, he’s gonna be a grizzly.”   

    GG then picked up Donny’s electronic calipers and the small cylindrical plug that held the bullet comparator. The base of the plug had a slot machined in it so that it could be slipped over one of the jaws of the caliper. The base also had a thumb screw for securing the plug to the caliper’s jaw. Donny saw GG eyeing the base of the plug.   

    GG:    If you look at how this plug has been machined, there is thicker metal on one side of the slot compared to the other. You need to make sure the thumb screw is inserted into the thin metal side so you don’t obtain a skewed measurement.   

    GG removed the thumb screw from the side of the plug that was thicker, and screwed it into the threaded hole on the thin side. He then cinched the plug to the lower jaw of the calipers and zeroed the caliper reading. He held the bullet seating depth tool back up so Donny could see how the outer sleeve of the tool had been machined at its tip.    

    GG:    The notch that has been machined at the tip of the tool is big enough to accommodate the jaw of a caliper.    

    GG then positioned the tool so that the end with the protruding bullet was placed into the bullet comparator attached to the lower jaw of the caliper, and the upper jaw of the caliper was placed into the machined slot on the bullet seating depth tool. The slot’s position allowed the upper jaw of the caliper to rest directly on the rim of the case that had been threaded on.    

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    “Pretty slick,” thought Donny. “A direct measurement of case base to bullet ogive.”   

    GG:    2.988 inches. If that measurement is repeatable, that is the one to which we will seat all the bullets for today’s testing. You try it, and you tell me what the measurement should be.   

    Donny replicated GG’s measuring sequence. His first reading was 2.993 inches.   

    GG:    This is a technique-based measurement, and I’m not surprised that yours is different. It really doesn’t matter what reading you get as long as you are consistent. The variability in measurement comes primarily from the variability in hand pressure on both the tool’s plunger and reading the caliper.   

    Donny continued to take measurements, and finally got three readings of 2.990 inches.   

    GG:    Good enough. Now we need to seat all the bullets to a base to ogive measurement of 2.990 inches.    

    D:    Before we do, I have a couple of questions. First, what is the reasoning used to select a bullet just touching the lands as the seating depth for this testing, and second, why aren’t we using cartridge overall length?   

    GG:    Good questions.   

    Using a test seating depth where the bullet just touches the lands is completely arbitrary. I’ve seen some really accurate final loads where the bullet just touches the lands or is no more than 5- to 6-thousands into them. I’ve never known of loads that weren’t at least semi-decent when the bullet just touched the lands. As far as not using the cartridge overall length, the length of the bullet between its ogive and tip can be highly variable. I’ve measured a variation of over 0.150 inches. That means the seating depth of base to ogive could have been off by the same amount if I had been referencing the measurement based on COAL. This Bullet from Hell was a lurker ‘match’ bullet in a box of 500 and could have gotten loaded as a 1000-yard match round. I’ll just leave it at that.   

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    GG had Donny clamp his press to the table and showed him how to set up the seating die per the instructions that came with it. GG noted that Donny had thoroughly cleaned the die and had applied a rust protectant.   

    GG:    I would suggest sneaking up on the seating depth, giving yourself about 0.002 inches above where you think you need to be. In your case, an initial target reading of 2.992. Seat them all as if you are trying to achieve 2.992, then go back and measure each one. With some you will actually get 2.990, maybe even 2.989. That’s primarily due to the inherent variability in case neck tension. If you are under 2.990, use your kinetic bullet puller to gently tap the bullet back out a tad. Then, try again with an adjusted die setting. The whole process is iterative, no set it and forget it. The seating depth needs to be uniform if you are to make any reasonable interpretations of this testing or future load development adjustments based on what the targets show.    

    Donny took GG’s advice and tried to sneak up on 2.990 by initially using a target seating depth of 2.992. He used the first round as a trial round to get 2.992, then seated all the others based on that die setting. He then went back and measured each round. As GG had predicted, the measurements bounced around between 2.989 and 2.993. A few were dead nuts on 2.990. He went back and adjusted the die for each of the non-compliers to get them all at 2.990. It was time-consuming, but he had satisfied himself that the seating depths were all ‘the same’.    

    GG handed Donny a dozen of his data sheets.   

    GG:    Keep records and targets. I have over 3 feet of paper stored in ten, three-ring binders. The data are an invaluable resource when you think you are up against a brick wall.   

    Don’t worry about filling in most of the columns. Just keep track of the basics that you see listed. Anything you can’t cipher on the form is associated with freaky reloading details that really aren’t relevant to the accuracy you’re trying to achieve.   

    D:    So my accuracy goal is achievable?   

    GG:    If there is no jerk on the trigger.   

    GG next produced a ruler and pen.    

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    GG:    Prepare two data sheets, one for each powder that we are going to test. Use the ruler to divide the ‘remarks’ section in half. That is where we are going to record the before-and-after extractor groove measurements.   

    Donny filled out both data sheets. As he did, he sensed the importance of cataloguing what he was doing. On the surface the process appeared simple and easily remembered, but he began to appreciate how it really wasn’t.   

    GG:      There is no precedent for measuring the extractor groove to estimate maximum pressure. Of all the reloading manuals I have read, only one gives an indication that a brass measurement on the extractor groove could potentially be used for that purpose. The technique shown is not specifically referenced to the extractor groove and is not specifically described in the text. The manual only shows a picture and a brief, one-paragraph narrative beneath the picture depicting a blade micrometer measuring the belt on a magnum case to determine if the case had expanded indicating that its maximum design pressure had been exceeded. The manufacturer that produced the reloading manual indicates that a measurement just in front of the extractor groove on a rimless case is equally effective. It makes no mention of measuring the extractor groove itself.   

    I’m not good enough to get consistent, repeatable readings using a blade micrometer. I have trouble enough using a regular jaw caliper. Plus, picking out ‘the’ measuring spot on the case for repeated measurements would be near impossible without some sort of jig fabricated to limit fore-and-aft variability. Errors due to case skew would also have to be accommodated by the jig. So, using a standard, traditional dial caliper in the extractor groove is nothing more than a Neanderthal approach based on expediency and taking measurements at the range. Some could argue the technique I am going to show you is like using a 5-pound sledgehammer and cold chisel to do brain surgery. I have no defense other than it works for me. My primer pockets are tight, there are no ejector pin marks on my brass, and the bolt handle lifts every time with no drama.   

    D:    Works for me.   

    GG got a target out of his field file, turned it over, then drew a very large circle with a pen. At the periphery of the circle, he used the pen to draw three tic marks, each about ½ inch apart.   

    GG:    What you are looking at is a blowup of the dial on a dial caliper. The three tic marks represent the 1000ths graduations on the dial. Let’s say that these represent

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8-, 9- and 10 thousandths on the dial. GG wrote an 8 next to one tic mark, a 9 at the next, and 10 at the last.   

    What you will find is that virtually all extractor grooves are slightly egg-shaped, so you have to make and record two measurements, a maximum and a minimum. You will also find that the indicator needle invariably falls between the graduation marks, indicating the actual measurement should be made in 10 thousandths. If you did, those precise measurements of a maximum and minimum extractor groove diameter would take a loooooong time, with the odds of repeatability slim to none for me. So, what I’m going to show you is my hand- grenade measuring approach.   

    Donny couldn’t believe what he had just heard. Mr. Anality was bailing on a measurement! How had the earth remained on its axis?!   

    GG saw ‘the look’.    

    GG:    Yep; guilty as charged. Sometimes meatball surgery gets it done. If it didn’t, there would be no market for duct tape.   

    What I do is visually divide the dial caliper’s graduations into 1/4’s. If the dial reading is slightly beyond the graduation line, I’ll add 0.00025 to the reading. If the dial reading is in the center between the two graduations, I will add 0.0005 to the reading. If the dial reading is not quite to the next graduation, I will add 0.00075 to the reading.   

    GG picked up one of Donny’s loaded rounds, then placed the extractor groove in the jaws of Donny’s dial caliper. He then began systematically rotating the case, calling out readings as he went. Donny figured GG gave the case at least three revolutions before he was happy with the maximum and minimum values.    

    GG:    As I recall, I finally decided on 0.40875 as a minimum and 0.4095 as a maximum.   

    GG picked up a pencil and marked the dial blowup where each of the readings had fallen based on the indicator’s relative position between the graduations.    

    GG:    Do you see how I am doing this?   

    D:    Yes.   

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    GG:    You try it.   

    It took more than three revolutions of the case for Donny to believe he had found the maximum and minimum diameters on the extractor groove. But he had matched what GG had determined.    

    GG:    Record those readings on the data sheet. You now know the pre-firing extractor groove measurements for the lowest powder charge of the fastest powder. Work your way up the powder progression, taking extractor groove measurements. Be advised, they will all likely be different. Brass is not perfectly uniform. Take your time. Be sure. If either or both of these maximum-minimum readings change after you have fired the round, you have exceeded the maximum design pressure of the brass.    

    D:    Either one?   

    GG:    Either one. Any change is not good.    

    And so it went. Donny went to the far side of the covered area where there was better light. He wanted to make sure of the measurements. GG had brought a clipboard that helped facilitate the process. As GG had said, it was rare when there was a common set of measurements.   

    GG:    In the remarks column, give each test cartridge a test number or letter. We will plot impacts on a scrap piece of paper so that the test number can be transferred to the actual impacts on the targets after you have completed the test session for each.   

    GG then picked up his battered field file.   

    GG:    Is your reticle a dot or a crosshair?   

    D:    Both. There is a fine dot in the center of the crosshairs.   

    GG:    What is the maximum magnification of your scope?   

    D:    Eighteen power.   

    GG scrolled through the folder’s compartments and produced two targets, both 8½ x 11 sheets of paper. There were 9 small squares, symmetrically oriented on each

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sheet of paper. It was apparent that a square template and magic marker had been used to make the targets. The open space inside of the magic marker lines that formed the squares was on the order of ½ inch.   

    GG:    The reticle’s dot should fit quite nicely within the squares. Aim small, hit small.   

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