Here is the method for building a super cheap Cartridge Overall Length Gauge.  This is not my idea.  I got this from an accomplished shooter and savvy reloader and I take no credit for this project.  

What you will need:

1. A fired case for the caliber you wish to make the gauge for.
2. Another case fired or unfired preferably for a smaller caliber than the cartridge being gauged.
3. A metal rod about 1/4 inch in diameter or smaller (smaller for .22 caliber guns).
4. A set screw of some type of about 8-32 thread or even 10-32.
5. A tap to match the set screw.
6. A 5/16 drill bit.
7. A drill bit of the correct size for your tap.
8. Another drill bit, the size of which is determined by the outside diameter of the second case described above (the one that is preferably smaller than the caliber for which the gauge is being built for).
9. A hand drill or drill press.

How to build it:

With your 5/16 drill bit, drill out the flashhole opening of the non-matching caliber case.  With the drill bit that matches the outside diameter of the non-matching caliber case, drill out the flashhole of the other cartridge case.  It should be a fired and unsized case.  Use a very small amount of 5 minute epoxy to glue the case neck of the non-gauge size case into the base of the other case.  Use just enough glue to fasten it securely and keep it as straight as you can.

Next, drill with your tap size drill through the side of the base of the non-matching caliber case as close to the rim as possible without actually drilling into the rim.  Once the hole is drilled, tap the hole with the matching tap and then start your set screw just deep enough to hold the screw.  Next, you insert your metal rod through the back of the gauge.  The parts should look as the picture below.  This is not a great illustration since it is for a .223 and I had no smaller than a .223 case and had to use two .223 cases.  It still works fine but would be easier to use the rear case were smaller.  For .22 caliber and smaller gauges, your metal rod must of course be smaller that 1/4 inch!

How it Works:

With the rod slid into the gauge, select the bullet that you wish to determine OAL for.

Place the bullet in the mouth of the oal gauge and insert the gauge into the chamber of the firearm as shown below.

Now, gently push the rod forward.  The rod will move the bullet forward until it is stopped by contact with the lands.  Don't force it!  Now, tighten the set screw to mark the correct postion of the bullet.  Next, remove the gauge from the chamber.  The bullet may or may not come out with the gauge.  If it doesn't, pop it out with a cleaning rod.  Next, with the bullet in the case mouth seated as deep as the rod will allow it, you measure the OAL from the base to the tip of the bullet.  Now you know absolute max OAL lenght for your chamber and you can load your ammunition with the bullet seated to arrive at the seating depth you desire.

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