Head spacing a semi-auto???

[jrplourde]

I started a new thread/question today "Short Brass". It came about because all of my
9mm brass was .005 to .010 shorter that the recommended trim length. A good fellow member
OKFCO5 cam back with an answer that most semi-autos head space from the extractor and a
short case is not a problem.

In this same vane I have wondered for a long time that the dimension from the bolt face to
the rim that would capture the case mouth was fixed; ie the tolerance in case length was
+/- 0.000. A short case would rattle around in there and a long case would be crushed.

Well I have shot this brass in all my 39s and a Luger with no problem so OKFCO5 is right.

Then what is all this stuff about head spacing on the case mouth and what need for a taper crimp?

This old boy is some confused. True! "Need input".

Bob

 

[jrplourde]

I started a new thread/question today "Short Brass". It came about because all of my
9mm brass was .005 to .010 shorter that the recommended trim length. A good fellow member
OKFCO5 cam back with an answer that most semi-autos head space from the extractor and a
short case is not a problem.

In this same vane I have wondered for a long time that the dimension from the bolt face to
the rim that would capture the case mouth was fixed; ie the tolerance in case length was
+/- 0.000. A short case would rattle around in there and a long case would be crushed.

Well I have shot this brass in all my 39s and a Luger with no problem so OKFCO5 is right.

Then what is all this stuff about head spacing on the case mouth and what need for a taper crimp?

This old boy is some confused. True! "Need input".

Bob

 

[OKFC05]

Simple. The headspace is defined as from the bolt face to the case head, set by the mouth of the chamber.

However, as a practical matter, many semi-autos have extractors that are fitted so as to positively hold the case in position for firing. Therefore, the case, held by the extractor, cannot move forward when struck by the firng pin. The gun then functions normally even with cases that are a bit short.

Similarly, a revolver shot with moon clips holds the cartridge in position regardless of the chamber headspace. This is why .40 S&W cartridges function perfectly in a 10mm S&W 610.

 

[Bruce51]

I'm afraid to say that you are getting incorrect information about headspace. I would suggest you look up the definition in the back of your reloading manual.

Headspace: The distance from that surface of the barrel or chamber that prevents the cartridge from moving further forward into the chamber, to the face of the breech with the action fully closed and locked.

The extractor may hold on to a case but it is not a headspace device substitute.

Rimmed cartridges headspace on the rim. Auto cartridges headspace on the mouth. Rifle cartridges headspace on the shoulder. Excessive headspace occurs when the distance from the shoulder,mouth or rim to the breech face is too great.

Auto cases that are slightly shorter will still function as long as the firing pin can hit them with enough force. I hope that this is a better explanation. Feel free to add or disagree. Bruce

 

[john traveler]

Bruce is correct with his definition of headspace and that the extractor is not a headspace device substitute.e

Auto pistols will still function reliably even though case length is shorter than specification because the firing pin protrusion is almost unlimited, and will still ignite primers even though headspace is say, 0.100" excessive. Designs that allow the extractor to hold a too-short cartridge and allow it to fire are notorious for breaking extractor claws. The Astra 400 comes to mind.

Also, the not-recommended practice of firing .380 ACP in a 9mm parabellum pistol is sometimes done when proper ammunition is not available. Sure, the extractor will hold it and the crtridge will fire and probably cycle the pistol, but you are risking extractor breakage.

 

[jrplourde]

Bruce and John and OKFCO5, thanks again. I am no longer confused and have the proper definition of head space.

Bob