Case Length and Headspace Considerations With the 9mm Luger

[Gnnut2]

I believe a headspace issue can result when using varying case lengths in auto-loading pistols. First of all, I know they are supposed be of uniform length, but mine are not. I have addressed the trim issues in a previous post asking for suggestions on how to remedy the problem. I understand that the case mouth will bottom out at the end of the chamber, which will result in headspace variances between the breech face and the case head. How much variation allowable is the question. Since I'm obviously talking about a short case length I'm wondering what effects this condition could have aside from shortened case life. Could it affect proper ignition due to primer strike, or does it happen so fast it doesn't matter? Can the extractor fail to lock onto the fired case and fail to extract, or is the extractor locked onto the case when the slide goes into battery? I'm just not certain about the function. Thank you for considering my question and I look forward to any comments and advice.

Regards,
Gnnut2

[Wobbly]

I haven't looked this up, but SAAMI defines those dimensions on case length which allow proper operation. It is not one length. It is a range of appropriate lengths. That is to say, the number has a tolerance on it.



This diagram says the acceptable case length is anything between 0.744" and 0.754".  Most of my cases fall around 0.746". I hardly ever see a case longer than 0.748".

When the case gets much shorter than those numbers, the primer is held against the bolt face by the extractor. If the extractor is in good condition, then the cartridge may still fire, but obviously this is not what the extractor was designed for, and if the condition continues the life of the extractor may be shortened.

Obviously, if the case does not head space on the end of the chamber, there is also a greater chance that the case will not seal the chamber during firing. Therefore, we could also expect a much greater amount of chamber pressure to escape toward the rear of the chamber, and the shooter. Such as situation might be: shooting 380Auto ammo in a 9mm Luger, or shooting 9mm ammo in a 40S&W gun. It happens all the time. Thankfully it rarely results in harm to the shooter, but the possibility exists and the risks greatly increase.

Bottom line: try to keep you case lengths longer than 0.744".


Hope this helps.

[Gnnut2]

Thank you for the reply and the SAAMI drawing.This clears up the chamber and cartridge dimension relationship for me. You have also answered my question regarding the extractor's role in securing the case when in battery, and as long as the case length is within the allowable 0.010 range. I can certainly see how a round that chambered much to deeply and fired could be a real bad deal.

Can the 0.010 tolerance have any negative effects on the taper crimp quality? I'm thinking it must not be a problem.

Regards,
Gnnut2

[Wobbly]

Can the 0.010 tolerance have any negative effects on the taper crimp quality? I'm thinking it must not be a problem.

You're correct. In 90% of the cases it's not really a problem. This because most Taper Crimp dies are simply sliding over the case mouth and forming the case largely with a radial force, not rolling it inward with an axial force as in the Roll Crimp.

At least that's what I see and suspect from using numerous brands of Taper Crimp dies. As in all things, there's a little bit of black magic going on, and you'd need to work for the die company to know ALL the voodoo at work.