Make Your Own Modified Case for Hornady O.A.L. Gauge
In this video, Forum member Erik Cortina shows how to create a custom modified case for use with the Hornady Lock-N-Load Overall Length Gauge (formerly the Stoney Point Tool). While Hornady sells modified cases for many standard cartridges, if you shoot a wildcat such as the 6mm Dasher or .284 Shehane, you’ll need to create a custom modified case*. And even if you shoot a standard cartridge such as the .308 Winchester you can get more consistent measurements if you make a custom modified case from a piece of brass fired in your chamber.
The process is straight-forward. Take a piece of brass fired in your chamber and full-length size it (with about .002″ shoulder bump). Then you need to drill out the primer pocket. Erik uses a mini-lathe for the operation, but this general process can be done with a drill press or other tools. Erik shows how to do this with a 0.290″ HSS (High Speed Steel) drill bit on a mini-lathe. After drilling the hole comes the tricky part — you need to tap the case with the precise 5/16″ x 36 threads per inch (tpi) right-hand thread that matches the male thread on the O.A.L. Gauge. This 5/16″ x 36 tpi tap is pretty uncommon, but you can order it from Amazon.com if you can’t source it locally.
If you use a mini-lathe, Erik suggests loosening the tailstock slightly, so it can float while cutting the threads. Erik also says: “Make sure you get the tap on pretty tight — it’s going to want to spin.” Erik turns the case at about 100 rpm when tapping the threads. Once the case and tap are rigged, the actual tapping process (see video at 6:00) takes only a few seconds. While the mini-lathe makes the tapping process go more quickly, the threading can also be done with other systems.
TIP: Don’t just make one modified case, make three. That gives you one for your range kit, one for your home reloading bench, plus a spare (since you WILL eventually lose or misplace one).
Here’s the Stuff You Need
| 5/16″-36 TPI Threading Tap The required thread is somewhat uncommon. You need a 5/16″ – 36 tpi Right Hand Thread Tap. If you can’t find it locally, Amazon.com carries the correct tap. Erik notes: “The 5/16-36 tpi tap is not a common size. I think Hornady did this on purpose to make it more difficult for the average guy to make his own modified cases.” |
0.290″ Drill Bit Erik uses an 0.290″ HSS “L” drill bit. (This “L” Letter Gauge code designates a 0.290″ diameter bit). A close metric equivalent would be 7.3 mm (0.286″). Erik says: “A 9/32″ drill will also work but it will be harder to run the tap in since the hole will be .281″ instead of .290″ with the Letter Gauge L bit.” |
Tips for Using O.A.L. Gauge with Modified Case
We’ve noticed that many folks have trouble getting reliable, consistent results when they first start using the Hornady O.A.L. Gauge (formerly the Stoney Point Tool). We’ve found this is usually because they don’t seat the modified case properly and because they don’t use a gentle, consistent method of advancing the bullet until it just kisses the lands.Here is our suggested procedure for use the O.A.L. Gauge. Following this method we can typically make three of four measurements (with the same bullet), all within .001″ to .0015″. (Yes, we always measure multiple times.)
1. Clean your chamber so there is no build-up of carbon, debris, or lube. Pay particular attention to the shoulder area.
2. Screw the modified case on to the O.A.L. Gauge. Make sure it is seated firmly (and doesn’t spin loose). Note, you may have to re-tighten the modified case after insertion in the chamber.
3. Place your selected bullet so that the ogive (max bullet diameter) is behind the case mouth. This prevents the bullet from “snagging” as you insert the tool in the action.
4. Insert the O.A.L. Gauge into your chamber smoothly. Push a little until you feel resistance. IMPORTANT — You need to ensure that the shoulder of the modified case is seated firmly against the front of your chamber. You may have to wiggle and twist the tool slightly. If you do not have the modified case seated all the way in, you will NOT get a valid measurement.
5. Advance the bullet slowly. (NOTE: This is the most important aspect for consistency!). Push the rod of the O.A.L. tool gently towards the chamber. DON’T shove it hard! Easy does it. Stop when you feel resistance.
6. IMPORTANT. After gently pushing on the rod, give the end of the rod a couple forward taps with your finger. If your bullet was slightly skewed, it may have stopped too far back. Adding a couple extra taps will fix that. If the bullet moves after the taps, then again push gently on the rod. NOT too much! You just want to push the bullet until it just “kisses” the lands and then stops. Do NOT jam the bullet into the rifling. If you do that you will never get consistent results from one measurement to the next.
* For a $15.00 fee, Hornady will make a custom modified case for you if you send two fired pieces of brass. Send fired cases and $15.00 check to: Hornady Manufacturing, Attn: Modified Cases, 108 S. Apollo St., Alda, NE 68810. More Info HERE.
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Tags: Bullet Seating, Erik Cortina, Hornady, O.A.L. Gauge, OAL, Reloading, Stoney Point Tool
Years ago I special ordered a 5/16″ x 36 tap for making these cases. Drilled the case out in a drill press and tapped them. Works well. Then I sent it to German when he still lived in florida and he made several. If you can get the tap your good to go,,,or use mine
why not just make the whole tool then you can use a standard size tap .
Just to pass on something I do along these lines. I have been a long time user of Wilson/Sinclair case trimmer and have continued adding their case holders as I add new calibers. These holders allow a cartridge case to easily be held in the lathe chuck whlile you drill and tap them for use on the OAL Guage.
Or you could just save time, money and effort and find the lands with the polished bullet method.
…and get accurate results, but who what’s that now-a-days….
Why not used a fire formed case then tap, that way the case fits the chamber perfectly and produces a more concentric diameter between the case OD and the tapped hole!
Also one would have to be real careful when chucking a case in the lathe and not over tighten crushing and or deforming the case.
Or you could send Hornady two fired cases from your gun and a check for $15.00 and get your modified case back in two weeks.
Editor: Good point. Here is the link for this service: https://www.hornady.com/store/Modified-Cases/
A very important step was overlooked!
you have to expand the neck to .001 over bullet dia. for it to slide in and out.
A full sized case will not work. It has too much neak tension. no one makes a expander of this size. I have proposed this idea to Sinclair’s. You have to turn your own from next size up expander.
I just use a taper reamers to open up necks until bullet slides through.
A 19/64″ drill is better than 9/32″, as it is slightly (+.006″) larger than the ‘L’ and it’s easier to tap. Since this will not bear any real load, having slightly shallower threads is just fine.
I recommend body sizing the case exactly like your loaded ammo, e.g. shoulder pushed back .002″ from fired or whatever.
This can be drilled and tapped without a lathe, using a drill press, with the case in an inverted body die, but the tap in the chuck is turned by hand (not the motor!), the press is just to align the tap vertically with the die / case, and the press handle is used to apply down pressure on the tap and advance it while you hand turn the chuck / tap. Easier to show than describe. It doesn’t require the precision of a lathe, in fact if you inspect the Hornady cases they’re not perfect either.
As I have looked into making my own cases, I have noticed the do-it-yourself instructions mention to full size the case. This contrasts to what Hornady mentions on their website for sending fire-formed cases. Hornady states to not resize the case.
Any ideas of why Hornady would not want resizing but others mention this step?
if the case isn’t sized right to chamber then you either have too much headspace or not enough headspace…therefore COAL will vary. If you don’t have enough headspace upon closing the bolt will force case to chamber effecting the measurement of the gauge. And bullets vary ogive that effects measurement. In my experience full length sizing eliminates troublesome gauging…. correct me if you feel I’m off base here.
Ben…..
mentioned in the video neck and body sizing not pushing back the shoulder
using fire formed case will cause the tool to be gun specific
full length will cause too tight neck tension
I use full length case install & remove bullet now it fits all