Author Topic: Determining Cartridge Maximum Overall Length  (Read 91 times)

Online Leo

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Determining Cartridge Maximum Overall Length
« on: August 02, 2017, 04:23:56 PM »
I was showed this method when I first started competing and it has been the way I determine my max COAL. I have been asked how I do this in the past and recently received a message asking to demonstrate this process. Here is how I figure out my max OAL.


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Offline IDescribe

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Re: Determining Cartridge Maximum Overall Length
« Reply #1 on: August 03, 2017, 04:07:39 PM »
Nice and clear. 

My normal process is to load a dummy a little too long.  Then, I plunk and spin test it, shorten it a few thousandths, plunk and spin test it, shorten it a few thousandths, and so on and so on until it plunks and spins with zero drag. 

I am going to test out this method the next time I buy a new bullet.


I have a question, though.  Instead of using the cleaning rod, making two marks, and measuring with calipers, why not just use the depth rod at the opposite end of the calipers, taking two measurements, and subtract?  It seems like you're putting an extra step in the process.

Offline IDescribe

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Re: Determining Cartridge Maximum Overall Length
« Reply #2 on: August 03, 2017, 04:15:40 PM »
On second thought, using the depth rod on the calipers might work with flat point bullets, but it would be too difficult to center perfectly with an RN, and it would exacerbate the previously mentioned problem with hollowpoints.

Offline noylj

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Re: Determining Cartridge Maximum Overall Length (and Plunk Test)
« Reply #3 on: August 04, 2017, 07:45:44 PM »
Your COL (Cartridge Overall Length) is determined by;
your barrel (chamber and throat dimensions) and
your gun (feed ramp)
and
your magazine (COL that fits magazine and when the magazine lips release the round for feeding)
and
the PARTICULAR bullet you are using.
What worked in a pressure barrel or the lab's gun or in my gun has very little to do with what will work best in your gun.
Take the barrel out of the gun. Create two inert dummy rounds (no powder or primer) at max COL and remove enough case mouth flare for rounds to chamber (you can achieve this by using a sized case—expand-and-flare it, and remove the flare just until the case "plunks" in the barrel and lock the die body down temporarily).
Drop the inert rounds in and decrease the COL until they chamber completely. This will be your "max" effective COL. I prefer to have the case head flush with the barrel hood (or a few mils higher than where the head of an empty case aligns with the barrel, as all cases are too short and I prefer to minimize head space). After this, place the inert rounds in the magazine and be sure they fit the magazine and feed and chamber.
You can also do this for any chambering problems you have. Remove the barrel and drop rounds in until you find one that won't chamber. Take that round and "paint" the bullet and case black with Magic Marker or other marker. Drop this round in the barrel and rotate it back-and-forth.
Remove and inspect the round:
1) Scratches on bullet--COL is too long
2) Scratches on edge of the case mouth--insufficient crimp
3) Scratches just below the case mouth--too much crimp, you're crushing the case
4) Scratches on case at base of bullet--bullet seated crooked due to insufficient case expansion (not case mouth flare) or improper seating stem fit
5) Scratches on case just above extractor groove--case bulge not removed during sizing. May need a bulge buster.