Headspacing on the Casemouth?!!


December 7, 2006, 07:34 PM
45 ACP out of H&K USP and 9mm out of H&K USP, do they really headspace on the casemouth or does the extractor set the cartridge position?

I've read both, how do I know the truth?

If you enjoyed reading about "Headspacing on the Casemouth?!!" here in TheHighRoad.org archive, you'll LOVE our community. Come join TheHighRoad.org today for the full version!
December 7, 2006, 07:42 PM
.45ACP headspaces on the case mouth. That's why you can't roll crimp it (even with cannelured bullets).

This is why I still think you should check brass length on .45ACP even though most people ignore trimming .45ACP. At least check for cases that are longer than SAAMI max length and chuck them. If loading for EIC pistol / bullseye, definitely trim all the the same length for consistency.

December 7, 2006, 08:42 PM
Get a case gauge. Size brass. Check all cases. If they go in their good enough for general shooting.
If you get real serious do like ocabj says and trim for more consistency.
If your this serious then clean & uniform primer pockets, use all the same brand brass, trim of course, chamfer case mouth, buy the best bullets, use the best powder, use competition dies, this is getting too complicated. :)

December 7, 2006, 10:03 PM
I don’t know the answer to the question, but could it be both. You might try a dummy round, trimming the case .10 short and chamber it and see if the extractor holds it. I would say that the extractor has to hold the case but not sure if it can if the headspace is wrong.

If you try this let us know what happened.

Car Knocker
December 7, 2006, 10:13 PM
Based on my own experience and research many years ago, in practice, cartridges headspaced on the extractor in the two pistols I used, a BHP and a Colt Commander (?). I trimmed some 9mm cases by 1/10", loaded them up when I made a run of that caliber, and they shot and extracted exactly the same as normal length cases. Did the same with .45ACP cases later with the same results.

December 7, 2006, 10:32 PM
I've picked up several spent cases at our range where some dummy has fired (yes, fired) .380 in a 9mm handgun and 9mm in a .40 S&W handgun. The cases were swelled up like balloons, but the rounds fired, and there was nothing for the rounds to headspace against.

In one case, the guy must have put several magazines of 9mm through his .40 pistol, as there was lots of swelled up 9mm brass laying around. The amazing part was that only one case started to come apart. The rest all held, which probably saved our rocket scientist from losing an eye from flying shrapnel and hot gasses. If the case doesn't swell enough to seal the chamber, all that stuff comes flying back at the shooter.

Several years ago I asked myself the same question concerning headspace in semiauto pistols. Since I've got them in lots of calibers, I decided to do an experiment. For starters, I tried .40 S&W in a 10mm Witness, similar to firing .22 shorts in a .22 Long Rifle chamber. Every round fired without mishap. Then I tried 9x19 in a 9x21 Witness, and again, every round fired without mishap. I then tried 9x19 in a .38 Super Witness, and again all rounds fired without mishap.

My conclusion was that at least in the Witness, the extractor was holding the case against the breach bolt tight enough for the firing pin to detonate the primer. All the cases I tried were the same diameter as the parent round, so there was no danger of the cases overexpanding and ruptering. My sole intent was to see if the cases did actually headspace on the case mouth, and in these particular instances, they did not.

I don't recommend anyone else trying these experiments, as the results in different guns, and with different ammunition, may not be successful, and may actually do harm to the gun and/or shooter. I only post this to shed some light on the original question.

Hope this helps.


Alvin in AZ
December 7, 2006, 10:33 PM
Cool one Car Knocker. :)

I'm going to try it with my B-92. :)

Alvin in AZ
ps- I'm a retired railroad signal-ape, you a RR car-knocker?

Car Knocker
December 7, 2006, 10:39 PM
Yup, but now retired.

Steve C
December 8, 2006, 02:35 AM
I think the answer is that in normal shooting the .45 acp case headspaces on the mouth. That is not to say that the extactor can not hold a case tight enough to still fire if it is too short for proper headspace against the mouth. The point is that the dimensionn isn't super critical, there's even noticable play with a round behind the extractor, try it out some time. With the slide removed from your pistol, slide a case behind the extractor and note how tight the extractor really holds it.. Cases are not uniform in base thickness either.

The simple test is to remove your extractor, chamber a round and see if it will still fire, it will by the way. This shows that the extractor isn't needed for the cartridge to headspace on. Firing short ammunition held by the extractor doesn't indicate that this is the normal or a designed operation of the pistol.

December 8, 2006, 07:27 AM
Steve C has it right.

Car Knocker
December 8, 2006, 01:30 PM
Another factor that will affect headspace is bullet design; the shoulder can be so far forward that a cartridge within proper OAL will headspace when the bullet shoulder contacts the rifling. I've had this happen with a Truncated Cone 9mm bullet - I don't recall the brand at this time. Had to reduce the OAL and the charge to remedy that interference. I noted the problem when I was chamber-gauging the finished rounds. I dug out a 9mm cartridge gauge and they failed that test also - the bullets were marked at the shoulder both by the gauge and the chamber when fully inserted.

The Bushmaster
December 8, 2006, 01:40 PM
Technically speaking orfeo...Almost all auto loaders are designed to headspace on the case mouth (possible exception: .357 sig).:)

One of the reasons that a lot of auto loaders based on the Colt 1911 will headspace on the extracter is because the US Army back in 1910 (?) required that the .45 ACP be able to fire upside down so the case extracter groove engages the extracter just as the expended case is ejected thus holding the next round inplace for insertion into the chamber.

December 8, 2006, 03:19 PM
Thank you all who took the time to respond thus far. ;)

This is what I'm getting at: Basically, all my pre-fired brass is already shorter than, or equal to, the S.A.A.M.I. minimums, and keeps getting shorter and shorter with each successive firing and reloading. If my cartridges are indeed headspacing on the casemouths and not the extractor, then the shorter and shorter cases will be causing my loaded bullets to sit deeper and deeper into the chamber as time goes on, until the point where the bullets will be engaging the bore as soon as they are chambered.

I have designed my loads to give a minimum bullet-jump (from chambered position to bore engagement) of .020 for 45 ACP and .038 for 9mm Luger using new Remington brass. This was done using only the actual barrel/chamber as the guage. The Remington UMC factory loads give minimum bullet-jump of .045 for my 45, and .117 for my 9mm in my H&K barrels. The accuracy gain by lessening the minimum bullet jump was appreciable for both calibers, but was drastically noticeable with the 9mm at all distances beyond 10 yards. I say minimum-bullet-jump as opposed to bullet-jump because if the cartridges are actually headspacing on the extractor and not the casemouth, then the bullet-jump distance will be somewhat higher. If it turns out that the cartridges are headspacing on the extractor and not the casemouths, then my bullet-jump distance will remain constant, regardless of case-lengths. . . whereas if they are actually headspacing on the casemouths, then my bullet-jump distance will change with the case-lengths. . . possibly even to the point, like I mentioned above, of prematurely engaging the bore upon chambering.

The Bushmaster
December 8, 2006, 04:02 PM
Although I have never see this so far, I would be more worried that the cartridge would go in far enough that the firing pin could not reach the primer. Not the bullet reaching the rifling first.

I have not seen a noticable difference in accuricy caused by the case length in my Colt .45 ACP or my 9mm X 19.

Jim Watson
December 8, 2006, 04:12 PM
There is no forward firing pin stop in a 1911. Maximum protrusion before the spring stacks or the taper binds is about a quarter inch.

George C. Nonte did some work years ago with the Astra 400, the "any 9mm" pistol, really made for 9mm Largo. He said that if, say, 9mm P headspaced on the case taper or on the extractor, that it shot fine. But if a rather small one got in ahead of the extractor and went the full 4mm to the chamber end, the firing pin would still reach and fire it with nasty results. Blown primers at best.

I found that a boxer primed Makarov case will run right through the 9mm Dillon with no felt difference greater than mixed 9mm P. I have been catching them when gauging finished ammo and pulling them down. But I got to wondering... the next couple that turned up, I loaded and shot. No discernable difference in the shooting. They fed, fired, functioned, and the bullet hit in the IDPA Zero.

So I would not worry about operational difference from shorter than spec .45 ACPs. As said, it might be worthwhile to get them all the same for NRA 50 yd slowfire. Especially if you use jacketed bullets. If you load SWCs right, they are seated to where the cartridge headspaces with the bullet shoulder against the origin of the rifling anyhow. I have Alton Dinan's report on that. He roll crimped the dickens out of them; it didn't matter where the mouth of the brass was, the bullet shoulder did the work.

December 8, 2006, 04:14 PM
Make some dummy rounds with a case at various lengths shorter (than your cases are now) and chamber them and see if there are marks on the bullet from the lands. You might want to take a marker and color your bullet, just to make any marks left by the lands easy to see.

The Bushmaster
December 8, 2006, 05:23 PM
Jim...I didn't know that. I think it's because I never took the time to look. Thanks. Now I'll look to see about that next time I clean that Colt 1911.:)

December 8, 2006, 08:29 PM
Can a 1911 be rigged to headspace on the extractor? Yes.
Is it designed to headspace on the extractor? NO. It is designed to headspace on the case mouth.
When I chamber guns I often check headspace with the extractor removed. headspace gauges do not feed from the magazine very well (they are a little blunt on the front for that) and I do not typically want to run the extractor over the rim of the headspace gauge. Steel gauges and steel extractors trying to slip are not conducive to keeping things in good condition.

December 8, 2006, 11:23 PM
Guys...ever measure how far out that 1911-type firing pin can move?

If it goes in the chamber and is centered, even if grossly short, it's likely to be hit hard enough to go "bang". Can trim cases back (as one poster stated) a good bit and still get reliable functioning...doesn't do accuracy a world of good, but they generally function. Standard .45ACp loads work at a pressure level that allows this kind of foolishness.

Other types of firing pins are a bit more pickly becasue their length of protusion is more limited...likely to have the pin more seriously decellerate in that "extra" 1/10th inch.

December 9, 2006, 02:45 AM
I was looking for something else and found this.

This is a Quote from CCI-

Q: Can I shoot 40 S&W ammo in my 10mm pistol? The case is identical except for length.
A: No. Both headspace on the case mouth. The shorter 40 S&W will not be properly supported in the 10mm chamber, so headspace control is lost. You'll get misfires, blown primers, deformed cases and, potentially, gas jetting from the action. Always use the correct ammunition for your firearm. Don't cut corners!


December 9, 2006, 06:00 PM
9mm definitely headspaces on the case mouth. How do I know? For one thing that's what my Speer book tells me. For another I found out what happens when you crimp the mouth a little too much. I just loaded 100 rounds of 9mm with 6g of Unique and Rainier 115g plated bullets with my new carbide Lee dies. I was using an assortment of different shells that I didn't check the O.A.L. Some of the longer ones got a little bit too much crimp so they would fall into the chamber. Apparently my Ruger P85 has a firing pin stop so the pin wouldn't even touch many of those rounds. There were about 6 out of 100. I'll probably set my die back a little bit and maybe check and trim any long cases.

If you enjoyed reading about "Headspacing on the Casemouth?!!" here in TheHighRoad.org archive, you'll LOVE our community. Come join TheHighRoad.org today for the full version!