"Headspaces on the case mouth"

[webby4x4]

I've seen this term a lot recently, and I want to make sure I understand this...

Essentially, this means that the case mouth (the top of the brass) stops against the end of the breach / beginning of the barrel, and stops the round from going any deeper into the chamber, right?

Whereas, some other rounds are stopped by some other means (e.g. a shoulder of a rifle round).


Am I right, or am I off on some other planet?


thanks,
Rick

[Bart B.]

There's three common types of headspace. They're checked with precision gages when fitting rifle barrels to receivers or reaming a pistol barrel or revolver cylinder for its cartridge. Measurements are typically from the bolt or breech face to the headspace limit point.

First one's with rimless bottleneck cases, They headspace at a point on their shoulder. For example, .308 Win. headspace a point that's .400 inch in diameter on their shoulder.

Second one's with belted cases, such as the .300 Win. Mag. They headspace on the belt in front of the extractor groove; it's typically .220 inch in front of the case head. A close cousin is rimmed cases, such as the .30-30 Win. that headspaces on the front edge of the rim just like .22 rimfire ammo.

Third are straight walled rimless cases typically used in hand guns; .45 ACP is an example. They headspace on the case mouth.

Like all rules, there's some exceptions. The .220 Swift is a rimmed case, but it headspaces on its shoulder.

The measurement from case head to case mouth is typically called "case length." In all cartridges except those rimless straight walled ones that headspace on the case mouth, there's s few thousandths clearance in their chambers for tolerances.

[Shoney]

We must make sure we all use the correct terms.

Breech - The chamber end of the barrel.

Chamber -
1. In a rifle, shotgun or pistol, the cavity at the breech end of the barrel bore that has been formed to accept and support a specific cartridge or shell.
2. In a revolver, the holes in the cylinder that have been formed to accept a specific cartridge.

Headspace - The distance from the bolt face (when the bolt is closed) to that surface in the cartridge chamber which stops the case’s forward movement.
For bottle-necked cases,, the measuring point is centered on the shoulder and is known as the datum line.
For belted magnum cases, the headspace is measured from the front of the belt to the head of the case. In practical terms, the amount of free movement a cartridge has in a closed chamber. This dimension is critical for the safety of the shooter, as well as the accuracy of the weapon system. Insufficient headspace hinders complete chambering; excessive headspace permits case stretching, separation or rupture, endangering the shooter.

All straight walled semi-auto pistol cartridge are supposed to headspace on the front lip of the case in the chamber. This is true for brass that is at the maximum length specified by SAAMI. You do not need to trim straight walled semi-auto pistol brass, because in reality, cases get shorter with each reloading. As the case gets shorter, the extractor holds the cartridge against the slide face where the firing pin can do its job.

Worst case scenario with short semi-auto pistol brass is that it will likely fall into the chamber, be pushed forward by the extractor, in which case it will not fire. I have 40SW cases that have been loaded 10-15 times, and 9mm and 45APC cases that are somewhere over 25 loadings or more. There have been no FTF's or other malfunctions.

[FlyFish]

There's three common types of headspace

.

Four - rimmed cartridges headspace on the rim.

ETA: just noticed that you included that in with the belted cases, but I really think it's sufficiently different to qualify as a 4th type - if anything, the belted magnums would be a subset of rimmed.

[Unclenick]

Six. You can also headspace on the extractor hook of a pistol, as Shoney described, which is fine with jacketed bullets but tends to detract from the accuracy of lead bullets as they are then entering the bore at a slight angle and are not hard enough to realign in the throat as jacketed bullets do before they start moving through it.

Then you can load the bullets out far enough to headspace on the bullet's contact with the throat. In old terms the throat is the whole entry to the barrel beginning with the freebore, while the tapered entry into the lands of the rifling was called the bullet seat. In the case of headspacing on the bullet, it is quite literally the spot the bullet seats against. Some semi-auto pistol chambers are too long to allow a bullet seated out that far to feed. Others have no problem (note that this changes with the shape of the bullet nose, too). For lead bullets, headspacing on the bullet produces the best accuracy and the least leading.

[Mike Irwin]

"Like all rules, there's some exceptions. The .220 Swift is a rimmed case, but it headspaces on its shoulder."

Cases that have rims but that headspace on the shoulder are referred to as semi-rimmed.

The .225 Winchester, the round that was supposed to replace the .220 Swift in the Winchester line up (but which failed because people wanted the Swift) was also rimmed and headspaced on the shoulder.



"All straight walled semi-auto pistol cartridge are supposed to headspace on the front lip of the case in the chamber."

For years the .38 Super Colt 1911 had a reputation for mediocre accuracy, at best. People just accepted it until the 1960s when custom barrel makers started making .38 Super barrels that headspaced not on the cartridge's vestigial rim (which is how John Browning originally designed it when he married the Colt 1900 with the .38 Auto cartridge), but on the case mouth.

People quickly got an idea for how fine the .38 Super's accuracy could be and the cartridge underwent something of a renaissance that has lasted to this day.

[webby4x4]

WOW! You guys are a vault of information!

Great descriptions - thanks guys.

While it's important to understand hadspacing, would it be a safe bet to assume that it's more critical for those "shouldered" bullets, such as 30/06, .243, .222, etc.? To ask a different way... when reloading my 9mm, .40 and .45ACP, it sounds like headspacing is handled by the OAL of the cartridge (not including the bullet), so measuring OAL is pretty straightforward.

One might conclude that headspacing on those shouldered bullets plays a more critical role, yes?

Thanks,
Rick

[FrankenMauser]

Like all rules, there's some exceptions. The .220 Swift is a rimmed case, but it headspaces on its shoulder.

Indeed.... Sort of. Winchester didn't like the size of the 6mm Lee Navy rim. So, they trimmed the size down to .473", to fit a .30-06 bolt face. This semi-rimmed design didn't leave much for the cartridge to headspace on, so they went for the shoulder. Between cutting production costs and trying to wow consumers; we ended up with a bit of an oddball cartridge.

I sure do love my Swift, though.

[Archie]

It is the case length that is critical in a cartridge that headspaces on the mouth of the case.

The only time I trim pistol brass is when I'm loading up ammo for a big match. Even then, it's probably of more value to me psychologically than in accuracy.

I must address Uncle Nick's comments. Yes, it is possible to catch a rim on an extractor and fire it. One can do this with a .40 S&W round in a 10mm pistol; I have done so. If one sets it up carefully, one can fire a .380 ACP in a 9x19 chamber in this way, but it doesn't work so well in rapid fire. 9x19 rounds will do this in .40 S&W pistols with rather mediocre accuracy and poor cycling.

One may also long seat bullets and use that to hold the round in place to fire. It works best in bolt action rifles.

Neither of these are methods of headspacing. No firearm is designed and intended to be fired in such manner. Yeah, it will sort of work; but neither are suitable for 'normal' conditions.

FlyFish, I'm with you. Belted cases are really just a modified rim set up.

In fact, most belted cases are better operated by setting up one's dies to only partially size the case and allow the cartridge to headspace on the shoulder. One can do the same with many rimmed cases with a substantial bottle neck. (.45-70 or .44-40 don't do well in this application. .30-30 will work quite well.)

[webby4x4]

I must address Uncle Nick's comments. Yes, it is possible to catch a rim on an extractor and fire it. One can do this with a .40 S&W round in a 10mm pistol; I have done so. If one sets it up carefully, one can fire a .380 ACP in a 9x19 chamber in this way, but it doesn't work so well in rapid fire. 9x19 rounds will do this in .40 S&W pistols with rather mediocre accuracy and poor cycling.

One may also long seat bullets and use that to hold the round in place to fire. It works best in bolt action rifles.

Neither of these are methods of headspacing. No firearm is designed and intended to be fired in such manner. Yeah, it will sort of work; but neither are suitable for 'normal' conditions.

La La La La La La La La La <-- That's me (the newbie) plugging my ears and saying "la la la" loudly so I don't hear you.

Someday, after a good 5+ years of reloading, some of that will make sense to me.

Thanks,
Rick

[Archie]

Pay close attention to the directions in the loading manuals and you'll be fine. And you will learn by doing.

That will make sense to you sooner than you think, I trust.

[webby4x4]

Just picked up another reloading manual today... that marks 11 total in my library. In fact... it's time to put down the laptop and pick up the manual.

Thanks,
Rick

[bcp]

A good headspace article:
http://www.cruffler.com/trivia-October99.html

Bruce

[darkgael]

I can't speak about other cartridges. As far as the .45 ACP is concerned, don't concern yourself overly with the length of the case. My experience over quite a few tens of thousands of reloads is that cartridge length is more important - too long and it won't fit into the magazine, and may influence your choice of bullet. I have, along with a lot of others if my reading through different fora is a guide, never had to trim a .45 ACP case. I just load them. Eventually, a case will split. Then it gets thrown away. I've never measured a .45 case; I may now, just to see. The ammo always works, in any case.
Pete
Oh, yeah - about using the extractor

Neither of these are methods of headspacing. No firearm is designed and intended to be fired in such manner. Yeah, it will sort of work; but neither are suitable for 'normal' conditions.

+1 about that.

[FlyFish]

In fact, most belted cases are better operated by setting up one's dies to only partially size the case and allow the cartridge to headspace on the shoulder.

Just so. I have a .300 Win Mag that I have always done this with and I've never had any issues with incipient head separation (nor have my .300 Win Mag cases!). I think the bad reputation that belted magnums have for case head separation are a direct result of actually allowing reloads to headspace on the rim.

[Bart B.]

Archie's comment:

In fact, most belted cases are better operated by setting up one's dies to only partially size the case and allow the cartridge to headspace on the shoulder.

...is a popular belief. But it was squelched back in the 1960's by both civilian and military teams and individuals shooting the best scores winning matches and setting records in 1000 yard matches with various belted magnums.

When the .30-.338 and .300 Win. Mag. was first used in long range matches, it was readily apparant they shot more accurate than the .300 H&H Mag that was popular since the late 1930's. Folks winning these matches had already learned that full length sized .30-06 and .308 Win. cases setting the fired case shoulder back a few thousandths produced the most accurate reloads. Doing this with the belted magnums was a flop. Brand new belted cases shot more accurate than traditional full length sized ones of any type; crush fit on the shoulder to setting the shoulder back to new case position.

The problem was found to be that swelled section immediately in front of the belt. Full length sizing dies didn't reduce the diameter there and that interfered with its contact point in the chamber. And neck only sizing was and still is a disaster with belted cases.

A few folks cut the bottom off their full length sizing dies as well as the shoulder and above making a die that would size just the body all the way back to just touching the belt. This "body only" die would be used either before or after a standard full length sizing die was used setting the fired case shoulder back enough to let the case headspace on its belt. Accuracy with resized belted cases finally got back to what new cases produced. Civilians shooting the best scores used this double die process. Military teams quit reloading fired belted magnum cases and just used new ones.

Larry Willis now makes and sells a collet die to do just this. www.larrywillis.com has good info. Check out his Belted Magnum Collet Resizing Die.

I have never got a belted case for three different 30 caliber magnums to shoot under 15 inches at 1000 yards unless they were double sized. My last test ran with the new barrel in my .30-.338 Keele Mag. was done with 15 new .300 Win. Mag cases resized and 15 ones of the same type that had been fired twice before developing loads and used these two types of dies reloading them. Both loads using Sierra 190's were shot in one 30-round test over about 20 minutes. Both groups were just under 7 inches at 1000 yards.