Making an OAL Gauge Modified Case

When you get serious about precision rifle ammunition, every detail counts. Using the right components, the right powder charge, and correct bullet seating depth are all critical for accurate loads. In order to “dial in” your seating depth its important to know the “to-lands” seating depth for a particular bullet in your rifle. When bullets are seated to this depth, the bullet’s ogive (curved portion that first contacts rifling) will be “just touching” the rifling. Most reloaders of precision rifle ammunition consider this “to-lands” seating depth to be the reference point or “datum” for load work-ups involving different bullet seating depths. In some cases, changing bullet seating depth by 0.010″ can have a meaningful effect on group size.

While there are multiple methods to determine bullet seating depth, perhaps the most straightforward is by using Hornady’s OAL gauge which uses modified cases that screw on to the gauge.

Hornady makes two of these gauges, straight (top below), and curved (bottom below):

These modified cases have a threaded hole on the back of the cartridge where the primer would normally be located. In this post I’ll demonstrate how to make your own modified case using a metal lathe (yes, you need a metal lathe!).

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My Lathe

For this build, I’m using a Precision Matthews PM-1440GT. I believe this is the best gunsmithing lathe you can by new in the USA (Taiwanese made, not chinese, 2″ through-spindle capacity, Japanese high-precision spindle bearings, etc):

5C Collets and Collet Chuck: Must-Have for Your Lathe

For this demonstration, I’m using 5C collets (typical for lathe use, with 1 1/16″ capacity) to hold a standard 7/8 14 die (Hornady 300 PRC die in this case). A 5C collet chuck or collet closer is my preferred work holding solution espeically for threaded parts that need to be clamped by the threads. Collets run very concentric (very little runout) and can clamp delicate parts securely. Furthermore, they are VERY quick to use (clamping and un-clamping work).

More information about my 5C collet setup:

Drills and Tap

The Hornady OAL gauge tool uses 5/16″ x 36 TPI threading. You can find these taps on Amazon, like this one:

For drills, I use a small drill to get the primer drilled out and the hole started. Any drill around 3/16″ should work. After this, you’ll need to drill to the appropriate size for tapping 5/16 36. An “L” sized drill will do the trick for this! (for full-depth threads, you can also use an “M” drill as I showed in the video, but threads will be shallow).

Drilling and Tapping the Modified case

You’ll want to select a case that’s been fired in your rifle that has a nice slip fit when you insert and remove a bullet of appropriate diameter. It’s OK if the bullet is just a tad loose in the neck of the case.

Start by removing the decapping rod and lock nut from your sizer die. Install the die in a 7/8″ collet, clamping on the threads. Push the case firmly into the die- the taper of the case will hold it securely. You can give it a light rap with a dead blow hammer to give it some extra clamping force.

Now do the pre-drill of the hole:

Assuming everything went to plan, install the “M” or “L” drill bit (L preferred) and drill out the hole again:

Now install the 5/16″ 36 TPI tap in the tailstock, shift the lathe to the lowest spindle speed, start the spindle, and push the tailstock to start the tap into the hole. You’ll want to use some cutting oil on the tap.

When the tap is most of the way into the case, stop the spindle, clamp the tailstock to the bed, and crank the tailstock quill (retracting) to use the tap as an extractor to pull the case out of the die (otherwise, this can be very difficult).

Done!

Now I’m ready to check my to-lands bullet seating depth for my 300 PRC full-custom! More on that shortly….

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Thanks,
Gavin

3 thoughts on “Making an OAL Gauge Modified Case”

  1. Frankford arsenal came out with a nifty tool
    for setting bullets off the lands. Two simple clamps on a rod from the muzzle end. Of course there’s the ole sharpie method. Tapping a fired case with the Hornady oal gauge is always a good method. Sinclair comparator inserts for CBTO measurements on calipers may be a better quality than Hornady’s.
    You got to love Gavin’s shop. I doubt it can get any better than that. He makes good videos too. Thanks Gav.

  2. I meant to type seating bullets off the lands,
    however setting works out just as well. These iPhone
    quick type auto word algorithms will get you know and then if you don’t double check- lol.

  3. Yes having a lathe would be very nice. may have to fid an older one for sale and pick it up. Love the videos you make very informative. question have you ever used the RCBS percision gauge it will do the same and more.

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