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Posted: 1/26/2014 9:45:15 AM EDT
| I just got this and tried using the bullet seating depth tool to determine the base to ogive distance for my AR. Each time, I would insert the tool by hand into the chamber and then slowly ease the bolt shut. Due to this gentle treatment, the extractor would not engage under the rim. Seems like this would give an inaccurate (too short) measurement. How do you Precision Mic guys use this on your black rifles? |
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The cartridge "simulator" that comes with the Precision Mic is pretty poor. It is unlikely its profile matches your bullet and it will yield questionable results for max OAL.
If you are going to use it, it is easiest if you remove the spring loaded things in the bolt (the extractor and ejector) because the tool has to sit flush against the bolt face. |
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I believe in an AR platform, that mag length will be shorter than you can seat to the lands. However, you can slip the case rim under the extractor and with some playing around, eventually get the cartridge base to sit upon the bolt face. You will have to keep pressure on the case to keep it in line as you guide the bolt forward. Do it several times until you get repetitive measurements. You're essentially reversing the extraction sequence.
Safe trips |
| Drop the lower from the upper. Buffer spring tension is too much to use the insert. Put the bullet insert under the extractor and hold tension against the ejector by pushing in on the insert to guide it into the chamber straight. Other hand, push the bolt in until it locks. Take a few measurements to make sure you're getting a consistent reading. Only if you're using 80-90 grain bullets are you going to get anything close to the rifling. For standard 55-77 grain bullets just load to magazine length. Exception being Hornady 75 A-Max which is too long for magazine length. It's a chore to remove the ejector unless you have the ejector insertion tool. Just for info, in my Wylde chamber a 77 Sierra MK overall length would be 2.316" oal just to touch the rifling. In two standard AR chambers the oal for Hornady 55 fmj bullets to touch the lands were 2.370-2.390" which makes the 55 grain bullet almost fall out of the neck. |
| The RCBS precision mic is great for measuring the case head to shoulder dimension when setting up a sizing die (I have one for each rifle caliber I load), but is practically worthless for determining seating depth. To accurately measure seating depth you need a tool that uses the actual bullet you're working with, such as the Hornady Seating Depth Gage. This is used with a bullet comparator. You must use the actual bullet because the comparator doesn't generally measure the exact point where the bullet engages the rifling; it actually measures the distance to a standard diameter (different for each model of comparator) on the ogive. This doesn't matter if you're using the actual bullet because you can still make a repeatable measurement and use it to seat bullets at a selected distance relative to the lands. I prefer the Hornady tools, but there are others that will also do the job, though I think the Hornadys do it well and are easiest to use. The RCBS dummy bullet is a PITA to use, and doesn't make the measurement you need. |
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The case shoulder contact with the shoulder mating feature, or conical section inside the AR chamber, registers the forward limit of travel of the cartridge before it is fired. Measuring ogive / land contact point on the bullet from a reference diameter resting on the case shoulder would be the proper way to measure this critical dimension. *
The RCBS precision mike feature for doing this uses an indirect method by establishing zero datum as the case head face of the simulated cartridge and then comparing it to the measurement from the real case head face (as matching datum) of assembled, sized, and reloaded cartridge. Imperfections on the real cartridge case head, variations from resizing, and where exactly that surface sits in the rifle chamber after bolt lock and right before firing, are uncontrolled variables introduced into the measurements intended to control jump distance of bullet to start of rifling. *The force of the ejector pin pushing on the case head face and/or the force of the firing pin against the primer are typically going to force the bottle neck cartridge forward, until hard contact with the case shoulder before firing. |
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So you get the big picture by now. You can use your Mic gage, but it is a transfer process that is for reference only. You can use it to track erosion and wear of your bbl.
The other tools are better for helping establish the "jump" of any particular bullet. You should find a bullet out of your samples that represents the standard sample. Many tip designs make this a statistical game. Once you have this sample, you keep it as a reference to use as a tool. The other part of your Mic gage set is good for checking the ogive length, but may still give some bias differences when compared to other tools that do the same job. Those shallow curved surfaces make it tough to get accuracy. |
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