RCBS Precision Mic - Headspacing and Optimal Bullet Seating Depth
It has been known for a long time among the shooting community that headspacing and bullet seating depth can play an important role in obtaining optimal accuracy for your particular rifle. In this article, we will look at the RCBS Precision Mic that will help up determine these important measurements. Before we begin, this article is only meant to be an illustrative guide and is NOT to be used as a substitute for the instructions that are included with the kit. Be sure to read, follow and understand ALL the directions listed in the manual.
Instruction Manual
It should be noted also, that the RCBS Precision Mic is caliber specific, this means you will need to purchase a Precision Mic for each caliber you want to perform the following operations on. Let's begin.
When you first open the green plastic case, you will notice the following parts.
Base
Headspace nut
Seating depth nut
Free bore tool
Instruction manual
Included Components
First we will measure the proper headspacing for our rifle. It is best to start with fire-formed brass (brass that has been used in your rifle at least once and is now "fire-formed" to your chamber). Insert the empty casing into the base of the Precision Mic tool, primer end first. Screw the headspace nut down until it comes in contact with the shell casing. There should only be a light amount of pressure applied as it comes in contact with the casing.
We don't want to crush or deform the shell casing by applying too much pressure when screwing the headspace nut down on the Precision Mic body.
Take a reading by looking at the numbers on the Precision Mic base and headspace nut and write it down.
Insert casing into Precision Mic base
Taking a Measurement
The measurements on the mic base are in .050" increments and the numbers and hash marks on the headspace nut are in .001" increments. Simply take a reading on each and combine the two.
In this example, we see that we have a .000 on the mic base and a .002" headspace nut. This means that our fire-formed brass is .002" longer than the "factory spec" for our caliber. This means that we will adjust our seating die to "bump" back the case shoulder by .002". We have just found the proper headspacing for this particular rifle. Now we will move on to finding the optimal seating depth.
Bullet Seating Depth
The optimal bullet seating depth is found in a similar fashion as when taking a headspace measurement. We will use the "free bore" tool, and the other nut (seating depth nut) that comes with the kit.
Loosen the set screw nut in the base of the free bore tool and extend it outward so there is plenty of length. The idea in extending it forward is to make sure it's purposely too long to chamber. We are trying to find out where the ogive of the bullet comes in contact with the lands of the rifle.
Loosen the set screw on the free bore tool.
Tighten the set screw so it's fairly tight and chamber the round in your rifle. You may not be able to get the bolt closed all the way, but just apply forward pressure until you are able to chamber the round. The free bore tool will be forced to adjust in length. This is what we want to happen.
Chamber the free bore tool.
Remove the free bore tool and place it into the Mic base. Insert the seating depth nut and screw it down until it comes in contact with the free bore tool. Take a reading as described above, and write down this number.
Insert into base.
In the photo below, you can see that we have a measurement of .131" or one-hundred thirty-one thousandths. We subtract .010" from this measurement for a final value of .121". This is means that we are going to seat the bullet .010" from the lands of the bore or .121" longer than factory specs.
Take the Measurement.
The steps described above should be repeated at least three times so you can be sure you are getting an accurate measurement.
Extend and chamber free bore tool.
Take a measurement using the Precision Mic and write down the value
Subtract .010" from the measurement to determine the "safe" bullet seating depth
Adjust your seating die and use the Precision Mic to check that your seating die is seating the bullet to the proper depth as measured above.
You have now successfully found the proper headspacing and optimal bullet seating depth for your rifle. Again, be sure to consult the instruction manual that came with your kit for further details and examples.
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