FMJ and CMJ same OAL data?

[seeds76]

can i use the same OAL data for both FMJ and CMJ? I assume it's pretty much the same, right?

[NoSteel]

Nope - Maybe.... Others will chime in here but to answer this correctly will need more information like whether or not bullet profile is the same, manufacturer, bullet weight, etc. In some cases you will find the bullet OAL is slightly longer so to get the same OAL you will be seating the bullet deper thus increasing your pressure slightly so a change will need to be made to your powder charge. I always re-work my loads based on the bullet type I'm using just to be sure where I'm at.

[twodownzero]

Pressure = Force / Area

[G-ManBart]

It depends. If the CMJ is the same exact bullet as the FMJ, but has a gas check style piece of copper that covers the base of the bullet, you can stick with the same charge. There might be a tiny difference because the base of the bullet is a slightly different shape, but there's more variation in individual cases, bullet weights, actual charge weights, etc that it's a non-factor.

If the two bullets have a different profile it changes things because they won't seat to the same depth using the same seating die setting.. To keep the pressure the same you have to seat both bullets so that they use up the same amount of case capacity; not the same OAL. If they use the same case capacity, and are the same weight, diameter and hardness, the pressure will be the same. R,

[Sarge]

can i use the same OAL data for both FMJ and CMJ? I assume it's pretty much the same, right?

Depends on alot I guess. BUT I know 124 MG CMJ and FMJ are the same except for the gas check. I load them the same with no problems. But I still verify things when I change from one to the other.

[njl]

I've gotten more or less the same results from PD 124gr FMJ, and MG 124gr CMJ. The seater had to be adjusted to keep COAL the same. There's never any guarantee that non-identical bullets will perform the same given the same bullet weight and powder charge...so when switching, it's a good idea to chrono the new load to see what it does.

[BradGannaway]

In cases where none of my manuals have data for a certain powder/bullet combo, I find something close (124gr FMJ/125gr FMJ, 200grSWC/200grJHP). I then lower the powder charge by about 10-20% (depending on powder) and seat the bullet until it will fit in the magazine.

I'll load a few like that, chrono them to see what they are producing, and see how accurate they are.

As long as I'm not running ridiculous speeds (1700 fps 9mm or 1400 fps .45s ), or pushing out primers, I'll seat a little deaper, increase powder charge, and chrono again.

I usually will keep doing this until I see signs of pressure or accuracy begins to degrade.

Actually, there are many more variables and indicators I look at, but those are my baseline analytics.

You have to be careful though, especially when working up loads with no data published.

Edited March 16, 2010 by Brad_G