determining optimum COL


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speedracer81
November 27, 2008, 04:32 PM
how do you guys determine the optimum overall length for a specific bullet?
I only have min oal in my load data.

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ants
November 27, 2008, 04:37 PM
Pistol, Revolver or Rifle?

Absolute basics:
Pistol: Whatever fits the magazine and feeds well
Revolver: Not longer than the cylinder
Rifle: Whatever fits the magazine and results in decent accuracy

rcmodel
November 27, 2008, 04:38 PM
Rifle:
1. Must fit in the magazine.

2. Get a good reloading manual for the brand of bullet you are loading. It will tell you the suggested OAL the loads were tested at.

3. Take a fired case and squeeze the mouth shut enough to keep a bullet in place.
Seat it long and chamber it in your rifle. The bullet will contact the rifling and push back inside the case. Then carefully extract it and measure it.
That is the maximum OAL for your rifle.

Somewhere between the maximum OAL and what will fit in the magazine is going to be it.

Revolver:
Seat to the bullet cannelure & crimp.

Auto Pistol:
Use recommended OAL in the brand specific load manual, then if it won't feed, make it longer until it will.

rcmodel

speedracer81
November 27, 2008, 07:33 PM
its a 124gr lead rn bullet in 9mm I'll be shooting them with a xd-9 tactical. I was reading in my reloading manual that some "bullet jump" is a good thing anybody ever experiment?

rfwobbly
November 27, 2008, 11:26 PM
Speedy -
SAAMI sets the maximum length of all 9mm Luger ammo at 29.70mm or 1.169 in. The reloading manual probably assumed you knew this.

I've never seen a data book quote minimum OAL, but then it makes sense seeing as how the biggest issue with 9x18 is chamber pressure. Since cartridge length contributes directly to pressure, setting the OAL longer for a given powder load would represent a reduction in pressure. In other words, allowing you to err towards the safe side.

In my experience, 9mm OALs shorter than roughly 1.080" start to generate feeding issues, since the cartridge has so much room to move around in the mag. So, especially with the RN bullets you mentioned, you'll want to be using an OAL out near maximum. The only reason not to use max OAL is with a severely reduced "sub-sonic" load, where you're having issues with the powder filling the appropriate volume of case space.

Hope this helps!

JDGray
November 28, 2008, 06:43 AM
I've never seen a data book quote minimum OAL

Lee manuals give this on every max load, so you don't have a pressure issue:)

straight-shooter
November 28, 2008, 07:02 AM
Take the barrel out of your gun. Load a dummy round with a longer oal than you expect. Drop it in the barrel. If it is raised above the barrel hood then seat the bullet slightly farther. Keep doing this till the bullet sets flush then seat it about .005 further. Test to see if it fits the magazine which at this point it should. You've successfully found your optimum oal for that gun.

This only works for semi-autos.

Now with the oal you've achieved, check it against your reloading manual and determine the proper powder charge for that oal.

rfwobbly
November 28, 2008, 08:39 AM
Some European 9mm chambers (like CZ) you have to do this on every new bullet type. I second Straight-Shooter's advise, except I do it ever so slightly differently....

Take several as-is pieces of fired brass. Fit your new bullets to this brass until you find one that's a perfect "push fit". Temporarily seat the bullet at about 1.20 inches. Push that "test cartridge" into the disassembled chamber until the cartridge goes all the way "home", and the mouth of the case seats on the chamber. Then carefully extract the test cartridge and measure it.

• If the test cartridge is still 1.20, then you can use the SAAMI max OAL of 1.169", but nothing longer.

• If the cartridge comes out shorter, then there's a physical obstruction pushing the bullet back into the case. Probably the bullet striking the rifling. That measure is the maximum chamber length. Subtract another .010" to get the max OAL for that bullet in that barrel.

Why the extra .010" ? Because you always want the bullet set back from engaging the rifling by at least .005", and an additional .005" because of typical loading press variations in finished OAL.

Hope this helps.

nksmfamjp
November 28, 2008, 09:36 AM
Simple explanation.

1) Load longer tham SAAMI by .02ish.
2) shorten by 1/8th of a turn until it fits in mag.
3) shorten by 1/8th of a turn until bullet doesn't touch rifling. Use sized case of the exact same case length for comparison.
4) shorten by 1/8th turn until the slow feed timing is right. This will require a trained eye. Basically, shortening the OAL increases feed speed. This also affects the pistols ability to control the cartridges path to the chamber. Too short and the cartridge must jump into the chamber and under the extractor. This requires "magic" which will not be 100% reliable. When slow feeding, this looks like a round which doesn't get under the extractor, but in stead tries to jump free! . . .out of the gun!

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