Posted by L.E. Wilson INC. on 5/28/2025 to
Reloading Tools
When it comes to precision reloading, every thousandth of an inch matters. A depth micrometer is one of the most important tools in your reloading bench for measuring shoulder bump and headspace movement during case sizing. But to get the most out of this tool, it’s essential to understand how to read it correctly.
In this guide, we’ll explain what a depth micrometer is, how it works, and how to read it for accurate and consistent measurements. We'll also highlight how L.E. Wilson’s Case Gage Depth Micrometer is designed specifically for precision-minded reloaders.
What Is a Depth Micrometer?
A depth micrometer is a precision measurement tool used to measure the depth of a surface from a known reference point. In reloading, it's often used in conjunction with a case gage to measure the distance from the base of a cartridge to the shoulder—helping reloaders determine how much the case shoulder is moved back during full-length resizing.
Why It Matters in Reloading
If the shoulder is set back too far, you risk over-sizing the brass, leading to excessive headspace, case stretch, or even failure. If it’s not set back enough, you may have chambering issues—particularly in tight bolt-action rifles.
A depth micrometer gives you the ability to monitor and adjust your die settings to achieve the ideal shoulder bump (typically .001" to .003").
Components of a Depth Micrometer
A typical reloading-focused depth micrometer, like the one from L.E. Wilson, includes:
- Micrometer Head – Marked with .001" increments for fine measurements
- Spindle – The rod that makes contact with the case inside the gage
- Scale Sleeve – Displays major measurements in .025" or similar intervals
- Locking Screw – Used to secure adjustments once zeroed
- Gage Block – Used to verify or reset the micrometer’s “zero” position
Explore the L.E. Wilson Case Gage Depth Micrometer
How to Set Up and Use the Depth Micrometer
Before using your micrometer for headspace measurements, it’s important to zero the tool correctly using the included gage block.
Step-by-Step Setup and Usage Instructions
- Zero the Micrometer
- Place the L.E. Wilson gage block on a flat surface.
- Set the Case Gage Depth Micrometer directly on the block.
- Slowly rotate the micrometer head until the spindle makes light contact with the block.
- Adjust the scale to read 0.100", this is our zero point to give you a plus or minus reading, ex. .096 indicates a -.004” from gage max.
- Tighten the locking screw to secure your zero.
- Insert a Fired Case into Your Wilson Case Gage
- Drop the fired case into the gage so it is fully seated.
- Drop the fired case into the gage so it is fully seated.
- Take the First Measurement
- Set the micrometer on top of the gage.
- Allow the spindle to rest on the case head and read the value.
- Record this as your baseline measurement off of an AS FIRED CASE – an as-fired case is a case that has just been fired in your chamber and NOT resized.
- Resize the Case
- Run the same case through your full-length sizing die.
- Run the same case through your full-length sizing die.
- Repeat the Measurement
- Reinsert the resized case into the gage.
- Set the micrometer back on top and take the second reading.
- Calculate Shoulder Bump
- Subtract the second reading from the first to determine shoulder set back.
- A typical shoulder bump should measure between .001" and .003", depending on your firearm and application.
You can find full instructions in the L.E. Wilson micrometer guide hereor follow along with our Video "How to Use the L.E. Wilson Case Gage Depth Micrometer"
How to Read a Depth Micrometer
Understanding the Scales
- The horizontal scale (barrel) gives you full thousandths (e.g., .025, .050, .075)
- The rotating thimble is marked in .001" increments
- Combine the reading from both to calculate total measurement
Example Reading
Let’s say:
- The barrel shows .100"
- The rotating thimble shows .095
Your total reading would be:
.100 + .095” = .005" drop in headspace, meaning you pushed your shoulder back .005” with the full length size.
.100 + .095” = .005" drop in headspace, meaning you pushed your shoulder back .005” with the full length size.
This tells you the depth from the MAX STEP of the case gage to the case head is -.005”, now you can adjust your sizing die to match your desired shoulder set back.
Using a Depth Micrometer with the L.E. Wilson Case Gage
Once zeroed, use the micrometer to measure how much your full-length sizing die is moving the shoulder. Here's how:
- Insert a fired case into your L.E. Wilson Case Gage. Measure the shoulder height.
- Resize the case using your full-length sizing die.
- Reinsert into the gage and measure again.
- Subtract the resized measurement from the fired measurement to calculate shoulder setback.
This process helps you fine-tune your die for safe, consistent chambering.
Final Thoughts on Reading a Depth Micrometer
A depth micrometer isn't just a nice-to-have tool—it’s essential for precise, repeatable reloading, especially in disciplines like long-range shooting, PRS, or benchrest.
L.E. Wilson’s Case Gage Depth Micrometer is purpose-built for reloaders, offering a simple, durable, and highly accurate way to monitor shoulder bump and headspace.
Ready to improve your reloads?
Shop the Case Gage Depth Micrometer at L.E. Wilson