Measuring Cartridge Overall Length Accurately

[Lexington1775]

Normally when I begin to load a new cartridge, I'll use 2 or 3 manuals to determine some basic dimensions; case length, overall cartridge length etc. I like to seat my bullets to a dimension measured from the bolt face when fully closed to the point on the bullet when it just touches the lands minus some number. This allows me to tailor loads to this specific rifle which generally shoot more accurately and more consistently.

I've never been really fond of measuring the overall cartridge length from the base to the tip because bullet length will vary .010 to .025. As the gap between the bullet and lands vary so will the accuracy or impact point of those rounds. Even the ogive dimension differs from bullet style so it's important to determine this length with the actual bullets you'll be using.

Several manufactures offer tools or devices to accurately measure this dimension however the one I see most commonly used is the Hornady Overall Length Gauge. The theory behind this gauge is simple and straight forward but requires a specially modified case.....and the 300 Blackout is not offered among them. I could send Hornady a few fired cases which they will modify for a price but all this takes time.

What methods are you guys using to accurately measure your chamber and how are you relating this to your loaded cartridge?

I have an idea which should work well as I've used it with my bolt guns but not on an AR...

 

[EddieCoyle]

You don't need a OAL gage or modified case to measure your C.O.A.L. You only need these tools to measure the maximum OAL in your chamber with the bullet you'll be loading.

To measure C.O.A.L., use the Hornady Bullet Comparator.

MassReloading - Rifle Cartridge Overall Length

To measure the chamber, I use the Hornady OAL gage. When I can't find a modified case, I improvise.

I use a sized case with a slit in the side (so a bullet will seat in it with a little bit of resistance). I put a drop of superglue on the bullet, start it into the slit case, then chamber it. I wait for the superglue to set, then poke the round out with a cleaning rod and measure the OAL with hte bullet comparator.

 

[Lexington1775]

Hey Eddie,

I agree...

My understanding to use the Hornady setup is I need a modified case shown below in the caliber I'm chambering.



I understand your improvised approach and I do something similar. I use a fired case and slightly squeeze the neck so it hold the bullet firmly yet allow the bolt to fully close and cam home.

I've also closed the bolt and slide a cleaning rod down the barrel until it contacts the bolt face...Measurement 1

I'll slide the bullet style I'm loading into the chamber then push it until it contacts the lands. The I take the same cleaning rod and again slide it down the barrel until it touches the bullet tip...Measurement 2.

Subtract measurement 2 from measurement 1 and this is your maximum C.O.A.L with this bullet. Using a "nut style" comparator allows me to derive the true distance from the case head to the Ogive of the bullet and I adjust my dies from there...

 

[EddieCoyle]

You need the Hornady OAL gage and modified case only to measure the max OAL of your chamber.

You use the bullet comparator to measure the C.O.A.L. of your ammo.

 

[Lexington1775]

You need the Hornady OAL gage and modified case only to measure the max OAL of your chamber.

You use the bullet comparator to measure the C.O.A.L. of your ammo.

Absolutely!!