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Make Your Own DIY Bullet-to-Lands Gauge
By Ron Colvin 6-7-19
The COAL or OAL is the abbreviation for the overall cartridge length and is also a measure of how close the bullet is to the lands in the rifle chamber. Precisely controlling the bullet clearance or “jump” to the rifling lands when the cartridge is fired is a critical parameter for the precision reloader. It is also important to anyone who is not using the standard OAL described in established loading manuals. Caution—determining this ‘Jump” is specific to the bullet and the rifle being used. Just because different bullets weigh the same does not mean they can be loaded to the same OAL! Loading longer than the specified OAL for that specific bullet or if using a barrel that has a short throat can result in high pressure dangerous loads. For example, loading with “zero” jump or with the bullet contacting the lands has been reported to generate 8,000 to 10,000 psi in higher chamber pressure. If you are at the max load of say 60,000 psi this could result in a 70,000 psi load!
To find out where the bullet is “just touching” the lands can be done with a tool available from Hornady or by the inexpensive DIY method described here:
- Take a fired sized case
- Use a Dremel Tool with a slitting disc ~.030” thick.
- Cut three longitudinal slots in the neck of the case making a bullet holding “collet.” This allows a bullet inserted in the neck to slide with some amount of friction. Adjust the length of the cuts to adjust the tension/friction. Make sure to de-burr the slots both inside and out especially the inside—a little polishing here helps.
- Assemble the “DIY Cartridge Gauge” with the OAL adjusted out longer than expected.
- Gently load the “DIY Cartridge Gauge” into the rifle camber and very gently close the bolt/action. The bullet will contact the rifling lands and be pushed back in the case establishing the bullet to lands contact OAL dimension for that rifle and bullet. Gently remove the “DIY cartridge gauge” taking care to keep the ejector from disturbing the bullet and gently measure the overall length with a set of calipers.
- Repeat the test 3 times and average the results. This is the “zero” dimension” specific to that bullet in that rifle.
- In general, take this lands contact OAL dimension and shorten it by .040 to .100” minimum to get the OAL you should load at. This is the bullet jump dimension. The shorter the jump the higher the pressure. Again—loading longer than the OAL (shortening the jump) prescribed in established Loading Manuals or “into the lands” can generate dangerous pressure levels and is best left for the highly experienced reloader.
- Every time you change a bullet and or rifle, this test must be done to be safe and especially if you are loading at longer than the established OAL.
Disclaimer– Reliance on any information provided herein is solely at THE RISK OF THE USER and the author or publisher will not be liable for any damages or injury arising out of the use, inability to use, or the results of use of this information or any third-party information, or the materials, services or products referenced herein.