Headspace on straight wall cartridges. What locates round in chamber?


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Haystacker40sw
July 17, 2013, 07:22 PM
I have just started reloading pistol rounds for my Sig Sauer SW40. All my reloading to this point has been target rounds for necked rifle cartridges (25-06, 30-06, etc). I am really trying not to over think this. (Weighing/batching bullets, "mic"ing primers, flame annealing necks, etc...) BUT, there is something that keeps gnawing on my brain that I cant seem to get a satisfactory answer for...
Straightwalled cases like the 40SW headspace off the case mouth. Crimping is simply to remove the bell/flare from the expander die, Neck tension does the rest. If I am reloading once fired brass of numerous different headstamps, I have cases that vary in length by .010" or more. Even if i hold my C.O.A.L. to say 1.135", it would seem that depending on the case length, my round could be in the chamber with the neck seating against the chamber register, the headstamp could be 10 thousandths off of the breech face, and my freebore could vary by the difference in the various case lengths. This makes my head hurt. One would think that the problem would be remedied by trimming all the cases to a set length for consistency, but depending on the chamber in your specific pistol all it would mean is that you have a uniform gap between the loaded round and the breech face.
So what am i missing? Does the extractor force the head against the breech via the extractor groove?
I know I said that I was trying not to get over persnickety with the details, but this seems kind of important to understand. Thanks in advance for the input...sorry for the suuuuper long question:banghead:

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Shmackey
July 17, 2013, 07:29 PM
More often than we would like, pistol cases do indeed headspace off the extractor. It is not ideal.

Centurian22
July 17, 2013, 07:31 PM
Wish I could be more help but the only pistol round I reload for is .32S&W Long and that headspaces off the rim (as far as I know). I'll be interested to hear the replys to your question.

Bmac1949
July 17, 2013, 07:33 PM
Excellent question! I've tried to get this right in my head and don't quite see it yet but this ought to do it. I just load MY .45 swc until there is only a thumbnail of the crimp grove above the neck and they shoot well. This will be good to know.

Shmackey
July 17, 2013, 07:35 PM
There's not a lot to it: if the case is too long, the round headspaces on the mouth and the slide can't go into battery. If the case is just right, the round headspaces on the mouth and things go as designed. If the case is too short, the round headspaces off the extractor.

Bmac1949
July 17, 2013, 07:43 PM
Dang! I knew that:) Just kidding. That makes sense to me.

Shmackey
July 17, 2013, 07:50 PM
To be fair, I'm thinking of 1911s and similar. Could be a different story in a very different design.

murf
July 17, 2013, 10:18 PM
welcome to the forum, haystacker40sw.

a couple things. first, the taper crimp helps keep the bullet from being pushed back into the case. many handloaders vary the amount of crimp when developing a load.

second, cartridge headspace (a misnomer, but basically means cartridge/chamber slop) on the 40 s&w cartridge varies with cartridge length (the shorter the case, the more slop). so, case length in this type of cartridge is very important to accuracy and firearm function.

a case longer than .850" will not have enough (or may not have any) cartridge/chamber slop. the slide may not close all the way (a safety issue). a case shorter than .840" will have too much cartridge/chamber slop and will not be very consistent (accurate) from shot to shot.

i suggest you trim all cases longer than .850" and toss all cases shorter than .840".

sorting cases by manufacurer is a good idea in case (no pun intended) wall thicknesses are different (causes another set of problems).

luck,

murf

Woolecox
July 17, 2013, 10:32 PM
I may be doing it wrong but I have loaded an shot thousands of pistol rounds. Mostly 9mm, 40S&W, and 45 ACP. I have never trimmed new or used brass. I really don't even mess with measuring it any more.

As long as there is a primer, powder, and a bullet in the case, they all go bang. I also make some very accurate loads with my Dillon RL550B these days. I run each and every round through a case gage when finished before they go in the box. This is mostly just to check for "Glock bulge" and burrs on the rim that would keep the round from chambering.

I shoot Glock, Sig, H&K, Khar, and several flavors of 1911. I'm not the ace of the base but I do compete with these calibers. No issues yet.

Not trying to be a smart arse, but this may be one of those problems that doesn't exist. Have you ever had an auto that would not function because of OAL.

Cheers,
Wooly

PapaG
July 17, 2013, 10:40 PM
Generally, if it is a rimmed case, it headspaces on the rim. If it is a rimless, it will seat on the case mouth....requiring a taper crimp. The 38 super is semi-rimmed and has had guns made to space on the rim, and some on the mouth.

beatledog7
July 17, 2013, 10:54 PM
first, the taper crimp helps keep the bullet from being pushed back into the case.

No. The taper crimp serves only to flatten the flare you may have used to help get the bullet started in without shaving. Proper neck tension controls setback.

astra600
July 18, 2013, 12:00 AM
Case lengths differ from the manufacturers as do cartridge OAL. The only straight walled case I load that needs checking is the M1 carbine. And that may just be related to my particular rifle. I don't check any of my pistol length cases, but do run them through a case gauge. I have also found if it doesn't quite fit the case gauge it has always fit in the barrel chamber anyway. I do not shoot .40S&W so can't comment on those. The rounds that fail the case gauge check get a close visual inspection, tried in the barrel chamber and put in a different container with a note. To date I have had no problems with their fit and function at the range.

The net is a great place for information. It can also overload you with so many details you don't know up from down, especially with firearms. Try to keep things simple, but keep informed of the basics and you should be OK.

Shmackey
July 18, 2013, 12:23 AM
I may be doing it wrong but I have loaded an shot thousands of pistol rounds. Mostly 9mm, 40S&W, and 45 ACP. I have never trimmed new or used brass. I really don't even mess with measuring it any more.

I shoot Glock, Sig, H&K, Khar, and several flavors of 1911. I'm not the ace of the base but I do compete with these calibers. No issues yet.

Not trying to be a smart arse, but this may be one of those problems that doesn't exist. Have you ever had an auto that would not function because of OAL.

I don't trim pistol brass either. It wouldn't be worth the trouble--not to mention that pistol brass actually shrinks, unlike rifle brass.

Because it shrinks, cases used multiple times will inevitably headspace off the extractor instead of the case mouth. Like I said, it's not ideal, but thousands of these rounds are fired every day and the world is still spinning. If I really want bullseye accuracy or OCD-level perfection in my pistol rounds, I'll measure everything and/or use new brass. I almost never do.

So the phenomenon of short brass and extractor-headspacing absolutely happens, but indeed it's not one of our bigger problems.

1SOW
July 18, 2013, 02:51 AM
a couple things. first, the taper crimp helps keep the bullet from being pushed back into the case. many handloaders vary the amount of crimp when developing a load.

No sir, It doesn't. Neck tension holds the bullet.
The taper crimp removes the bell sometimes needed to seat the bullet.
Too much crimp can deform or sometimes damage the bullet, as in the case of lead or plated bullets and sometimes even jacketted if it's overly crimped..

Haystacker40sw
July 18, 2013, 03:34 AM
Thanks for all the input guys. I guess the gist I'm getting is not to sweat it like I do in my rifles. Both the Sig and my wifes Beretta PX4 were bought for defense. So the shooting is quite different than the measured breathing, pulse control, peep sighted, zen "be the trigger" shooting that I've been accustomed to. Its actually pretty fun shooting 3 shot rapid fire at targets 15-25 ft away. I just need to be sure that the rounds we are using for practice will chamber, extract, and not blow up my guns. The headspace questioncame up because i've seen some fellas at the range with raced out 45 ACP punching out VERY tight groups @50 yds. Its difficult to understand that kind of consistency when the headspacing seems so up for grabs. I will load some up and sneak up on a good charge for both guns. Also guess I better get a case gage rather than the "plunk"test I've been doing. Thanks again for the help :-)

gamestalker
July 18, 2013, 05:26 AM
I'm pretty much an odd ball when it comes to reloading, in that, I trim all my brass to the same lengths, I don't load anything but jacketed bullets, and I don't bell or use any crimp at all on rimless cartridges, such as 9mm and .40 cal.

My method doesn't require that you trim your brass either, that's just a personal thing of mine. What I do, is when loading any jacketed bullet, I chamfer the inside of the mouth evenly, set the bullet on top, which will set up perfectly straight on trimmed brass, and then just seat. No having to crimp or bell the case mouth at all. And the result is, I get the maximum degree of neck tension obtainable and I don't have to mess around with crimp adjustment or the bell, and it saves some time and frustration. I've been doing it like this for a number of years with nothing but excellent results.

I don't think this will work for anything but jacketed, bullets, maybe TP plated bullets but I haven't tried it as yet.

GS

Haystacker40sw
July 19, 2013, 01:15 AM
Excellent post gamestalker. Never really thought about it but the logic is really straightforward. I have a couple thousand FMJ 165 gr Montana Gold bullets that would work great for this. Might have to ditch the bullett feeder. We'll see. After chambering the reloaded rounds by force loading with the slide I get about .005" bullet setback. Not a deal breaker, but would rather have zero. Skipping the bell would give me more neck tension.

Thanks again!

Bad Flynch
July 19, 2013, 05:53 PM
If you want to see mechanical drawings of typical headspace dimensions for cartridges, see the drawings at www.saami.org . Those are the official drawings and you can deduce how it works with a little self-study.

However, simplified definitions of headspace are as follows:

Chamber headspace is the dimension from the breech face to that point in the chamber that stops the forward movement of the cartridge. Cartridge headspace is that dimension from the head of the cartridge case to the point on the cartridge that stops its forward movement in the chamber.

Cartridge and chamber headspace are not the same, as there must be some working allowance for feeding, manufacturing tolerance slop, dirt, and so on.

Since you are interested in calibers for auto-loading pistols, we will stop at those. Generally, these fit into three or four categories: Rimmed, which are rare; semi-rimmed, which are seen occasionally; and rimless, which are the most common. Bottle-necked cases can be used, too.

Rimmed cases headspace extends from the head of the case to the front of the rim. Cases like this, say the rare .38 Special autoloader (not AMU), shoot well, but are difficult to manage in an auto-loader.

Semi-rimmed cases headspace extends, likewise, from the head of the case to the front of the rim. Because of their abbreviated rim size, they, like the .38 Super Auto, are easier to manage in a semi-auto pistol.

Bottle-necked rimless cases, like the .357Sig headspace like a bottlenecked rifle case. The cartridge headspace extends from the head of the case to a point on the shoulder arbitrarily defined as a reference "datum line." They function very well in auto loaders, but are generally miserable to load properly. They can, however, be loaded properly if enough care is taken.

Straight-walled, rimless cases, like the .40 S&W and the .45 ACP will headspace from the case head to the case mouth. As has been noted here, there is always some slop and the fact that these cases tend to shorten with use, does not help. These cases shorten because the case expands when fired, pulling the mouth of the case back and typical sizing dies do not size enough of the case to recover the lost material. While it is necessary to have some working tolerance, we frequently have way too much.

Target shooters have, in the past, been known to seat lead bullets out to the point where the lead bullets barely engage the rifling. This should, and does, take up the extra space, but has the nasty tendency to cause problems. Sometimes, they will not fit the magazine or will not feed. Sometimes, the lead bullet will stick in the leade of the chamber and prevent proper chambering, especially after the build-up of firing residues. Sometimes, the bullet will stick in the leade and prevent extraction of a loaded round or pull out of the case when a live round is extracted.

As noted by another contributor, the effective fit of the case is often extractor limited. Given the slop with which extractors are fitted, this seems to be an unacceptable situation in a target gun. Target guns, with their very tight chambers typically suffer less from case slop, but then the end accuracy requirements are much greater, so no help comes here. The advice to sort and trim your cases so that the lengths are as consistent as possible is good advice and unless you are willing to buy new cases for every loading, it is the only workable solution.

.45ACP cases are known to shorten to such a degree, that some shooters simply discard them when they get too short. However, there may be a solution at hand to resurrect some cases: Both Lee and Redding are making push-through dies that will size every bit of the case. These are available for .40S&W and for .45ACP and maybe others. Sizing the case all the way past the head, from "head to toe"(so to speak), might get back some of the shrinkage. I have yet to try my dies made for this purpose. The Lee setup is cheap enough to try without losing much.

Hope this helps.

TRX
July 19, 2013, 06:30 PM
Theoretically the 1911 headspaces off the case mouth. The thin ring of brass at the mouth at one end, the case head at the other, case in compression, and the "hood" (tab at the end of the barrel) is NOT supposed to touch the face of the slide.

This means the gun locks up slightly different as case lengths vary. Whether this makes any difference, I don't know.

Some 1911 copies have the hood contacting the breechface, the cartridge floating slightly in the chamber. I think that gives a more repeatable lockup than the original Browning design.

Potatohead
July 20, 2013, 01:59 PM
Thanks very much for that post BadFlinch, even though I'm still confused. I am really not understanding this "such and such headspaces off the case mouth" stuff, though I'm trying.

GLOOB
July 21, 2013, 06:58 PM
As others have already stated, a straight wall pistol round can headspace off of the case mouth (rarely), or off of the extractor (very common).

They can also headspace off the ogive of the bullet against the rifling. This is common with many peoples' cast reloads. This can help with accuracy. Whether this is a result of increasing initial pressure (and, perhaps, making it more consistent) or just by virtue of a larger area of unmolested bullet riding the bore, I'm not sure.

And some pistols have such tight chambers, they can actually headspace to some degree off of the outside of the case mouth; you may have to taper crimp slightly beyond removing the flare just to get the round to drop all the way in.

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