Exactly what does the term headspace on the mouth of the case mean, and what effect does crimp have on loads that headspace on the mouth of the case?
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Exactly what does the term headspace on the mouth of the case mean, and what effect does crimp have on loads that headspace on the mouth of the case?
when you headspace on the case mouth it means that you want your cases to be the same length.
what is happening is that the case enters the chamber and stops at the end of the chamber,by the case hitting there.
if you roll crimp a case that does this it will/can enter untill the crimp or the boolit engaging the rifling stops it.
both situations can raise pressure, or affect your accuracy as will the longer shorter case.
some guns will hold the shorter case by the extractor allowing it to still be fired but the case can be tipped in the chamber area and your bullet will enter the rifling at a shallow angle.
this is why your pistol loads are taper crimped,there are some special cases where the 45 acp is roll crimped slightly but this is an accuracy thing and is pretty specialized.
Problem is that most auto cases are shorter than the chamber - measure and find out. On the .45, the main thing the crimp does is take out the bell put in previously. A light roll crimp does fine, and so does a taper crimp. Just seat the bullets out far enough to contact the leade of the rifling - when dropped into a dis-assembled barrel, the loaded round shoud be proud a smidgen. The slide slamming home will take care of business.
This and that about calibers that headspace on the mouth of the case. Have questioned about it myself as I thought it was really important. Until I really understood the 45ACP headspaced on the mouth of the case I just loaded them up and fired them without a problem. After understanding, or questioning the fact the 45 should be trimmed to the correct length because it headspaces on the case mouth most replied they never trim their 45 cases and never will. I have never had a problem not trimming to specs. I am still up in the air about this fact myself and would like a clear answer.
Most, if not all, rimless auto pistol cartridges are designed to headspace on the mouth of the case. If you look inside the chamber you will see the distinct point where the cartridge case mouth is supposed to be stopped. This is a 90 degree shoulder, rather than a taper. That being said, a lot of people use the end of the leade/ start of the rifling lands to headspace against. My biggest problem with this is that any change in bullit nose shape or seating depth, whether intentional or accidental, will change the headspace. I would rather have total control of the headspace by reloading the cartridge to fit my chamber. My preferred method of doing this is to get a good cast of my chamber and trim my cases to fit those measurements.
Most loaders never trim pistol cases. I know I don't. In theory, they headspace on the case mouth, but in practice, the extractor holds the case in place for the firing pin strike. The test of this is to remove the extractor and see for yourself that you will get mostly FTF's when you do as the round is pushed forward until the case mouth hits the front of the chamber and the firing pin cannot reach the primer. IME, the only time it may be critical is when you have a SA revolver chambered for a pistol round (like a Ruger Blackhawk with the interchangable .357/9mm cylinders), where the case actually does headspace on the mouth. Then you have to be careful your cases are long enough or you will get FTF's as you do in a pistol sans extractor. (The "star" extractor in a DA revolver will usually hold the cases in place for firing like that of the pistol.)
or you get a case too long and it binds the cylinder.
same thing can happen to a pistol they will come to a halt short of battery and jam up.
one way i keep my cases straight in the chamber is to use a cast boolit large enough to just slide in the chamber this helps align the boolit to the bbl. [this also gives resistance to the firing pin srike]
same thing in a revolver i neck size my revolver brass [where the boolit goes only]
so it is held in better alignment in the front of the cylinder and supported in the rear by the fireformed case.
i have to thumb seat most of my cases in my revolvers.
Ed Harris called it end-play, that movement that exists due to the fact that the .45ACP case (for example) is several thousandths shorter than the chamber. Excessive end-play causes ignition variance, according to Mr. Harris. He suggests seating the boolit out .015-.020 so as to contact the leade and remove the end-play. I seat mine out a little further, maybe .050. Chambering will engrave the rifling on the boolit and end-play will be minimized.
I have never had a problem loading 45ACP, 40S&W, or 9mm which all headspace on the front of the cartridge case and I never trim those calibers. Most action pistol shooters, who may shoot many thousands of rounds in a year, don't trim either.
I must take exception to the practice of headspacing on the bullet though! If there is no space between the bullet and rifling then pressures can soar and you could find yourself with a ruined pistol... or worse! Taper crimping to just the size of the factory specification case mouth diameter for the particular round is the recommended way to go. If the case has been properly full length resized anyway, this will insure enough bullet grip to prevent "setback" of the bullet while feeding and will not create an overpressure situation in the gun. You can load longer than the recommended cartridge OAL with no problems as long as the bullet is not actually forced into the rifling when feeding which is easy to check by loading a few rounds and dropping the slide on them then cycling them by hand and inspecting the unfired round to see if there are rifling marks on the bullet. If there are rifling marks seat them a little deeper until they no longer show any contact with the rifling. Let's not blow things up!
And that is a process that is called smoking the bullet. I would still like to see some type of pic or a reasonable post about cases like the 45, 9mm or 380 that is supposed to headspace on the mouth of the case and a resonable response as to why you can chamber the cases longer than SAMI Specs. I understand you may have to slug. But so many state they never trim their cases it still has me wondering.Quote:
Originally Posted by JustSomeGuy
Most "service guns" have oversized chambers, for our own good of course. As we are talking about autoloaders, their barrels often have a sloppy fit allowing the chamber to "stretch", and their firing pins have enough reach and energy for short cases.Quote:
Originally Posted by armyrat1970
Try pushing your 1911's barrel forward, or insert a gauge/toothpick between its hood and the breech face.
Trimming is not necessary because straight auto cases don't seem to stretch; ... doesn't make sense with the metal displacement thing during sizing, but the fact is that I even stopped having to trim revolver cases when I stopped crimping them.
Your case trimmer might tell you otherwise if it headspaces from the front of the rim, as they are battered by the ejector, for example.
I think headspacing should be functional rather than, say, academic ;). It should avoid misfires and damages but allow field use.
("In practical terms, headspace is the clearance allowed between the base of the cartridge case and the face of the bolt", http://www.fulton-armory.com/headspace.htm).
About SAAMI specs, you should have one set for cases and an other one for chambers, which might support my view.
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