A term I have heard since the days in the army, I have never fully wrapped my head around headspace.
Do you have any links to explain headspacing and the importance of it.
I searched THR and could not really find it. Maybey you know the thread I might be looking for.
Much appreciate your comments.
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September 30, 2011, 11:41 AM
I assume what you really want is to know is the practical effects of how headspace affects what you should do when you size your reloaded cases rather than the techical stuff you've found in books? Okay, but forget what you "know" for a few minutes. Otherwise you will remain confused. I don't often write web essays but we ex-GIs got to stick together! :)
Where most of us 'experts' (and books) get new guys more confused than helped is by jumping directly into how and where headspace is measured for different types of cases; fergit it. None of that stuff is relivant to what you need to know to properly reload. Actually, the applied info on headspace is actually pretty simple. Technicially defining what the books say is better left in the books because it only matters to geeks like me.
I'm assuming you do know that headspace, as such, is in the chamber. That's set/machined within a specified tolerance range during manufactor and we can largely ignore it. However, it's obvious our reloaded cartridges have to snuggly fit in our chamber. The 'head' of our cases must be controlled in the chamber or things start messing up - either we can't close the action or cases stretch too much for safety. We need to control the cartridge so it fits the chamber in such a way that the bolt can be closed and the cartridge not be too loose/sloppy a fit. That's really all we reloaders need do or understand. Alright?
Cases expand in diameter when fired. Resizing squeezes the diameter in and, just like squeezing a roll of PlayDough in our hand, the case gets longer as the die squeezes. That squeeze pushes the shoulder forward, eventually making it too long to chamber unless we push the shoulder back to near where it was before we sized. Moving shoulders back maybe 3 thou for an auto loader or 0 to 1 thou for a bolt gun is plenty of set back to maintain all the "headspace" chamber fit we need.
Bottom line, all we need to do is set our sizer die to do is restore the sized shoulders to match the fired shoulder location so there will be no slop - no 'headspace' - to worrry about. We can do that by carefully adjusting the sizing die down - (correctly, more below) - and test fitting the sized cases in the chamber as we go. Precision measuring tools like Hornadys case headspace tools on a dial caliper or RCBS' Precision Case Mic make it easy.
Many times I read well intended folks saying to screw your sizer down to touch the shell holder plus "X" more turns. Nonsense! That 'formula' assumes all rifles, dies, shell holders, presses and cases are the same; they are not. Yeah, it makes ammo that will loosely chamber and fire but it rarely makes ammo that actually fits our rifle! I also commonly see suggestions to get a Wilson type "drop in" cartridge/case gage but that's also nonsense, it makes finished ammo that fit and fire in any rifle ever made for that cartridge but there is nothing custom fitted about it. And I VERY OFTEN see BAD suggestions to turn your sizer down in 'small' increments like 1/8 or 1/4 turn; NO! Full min. to max. headspace, from head to shoulder, in most bottle neck cartridges is only 6 thou. Even a 1/4 die turn potentially moves the shoulder about 18 thou and that's fully three times the total tolerance range! A 1/16 turn moves the die about 4.5 thou and that's over two thirds of what we should aim for, all meaning small die steps should indeed be small and knowing that helps explain why using a precision case gage of some sort can be important.
Note that I've not mentioned how to measure anything from a rim or belt or certain kinds of specifications like shoulder datum lines. All that's interesting trivia for some people but it has NO practical meaning to a reloade; just work off the shoulders and make them fit YOUR chamber and all will be well at your range.
Also note that I've deliberately not addressed any straight wall (auto pistol) cases that 'headspace' off the case mouth but that's because other than trimming the occasional over long cases (rare) there's nothing we need do about them.
Link for bottle neck cartridges > http://www.brownells.com/.aspx/lid=12897/GunTechdetail/Gauging_Success___Minimum_Headspace_and_Maximum_COL Different types of headspace > http://i338.photobucket.com/albums/n420/joe1944usa/Firearms%20%20and%20%20Reloading/cartridgetypes.jpg Cartridge & Chamber drawing with measurement at SAAMI > http://www.saami.org/specifications_and_information/index.cfm?page=CC
September 30, 2011, 12:54 PM
To keep the brass in 1 piece on firing. http://i338.photobucket.com/albums/n420/joe1944usa/th_reloading_2.jpg (http://s338.photobucket.com/albums/n420/joe1944usa/?action=view¤t=reloading_2.jpg) http://i338.photobucket.com/albums/n420/joe1944usa/th_6ppc.jpg (http://s338.photobucket.com/albums/n420/joe1944usa/?action=view¤t=6ppc.jpg) http://i338.photobucket.com/albums/n420/joe1944usa/th_338.jpg (http://s338.photobucket.com/albums/n420/joe1944usa/?action=view¤t=338.jpg) http://i338.photobucket.com/albums/n420/joe1944usa/th_caseseparation.jpg (http://s338.photobucket.com/albums/n420/joe1944usa/?action=view¤t=caseseparation.jpg) Click photos for larger view.
September 30, 2011, 03:27 PM
Headspace is machined into the firearm. As long as cases are sized properly, if the firearm has proper headspace, we are OK.
We can induce "artificial" or "working" headspace in a rifle chambered in a bottle necked caliber by excessive sizing, pushing the shoulder too far back, and giving the loaded round too much back to front play in the chamber. While the mechanical "headspace" is just fine, we have induced excessive artificial headspace by improperly sizing.
Technically it is really excessive case clearance we have induced, because the rifles headspace, technically, is OK as far as headspace is concerned, but the situation is the same as shooting a properly loaded round in a rifle with excessive mechanical headspace.
We have induced excessive headspace by improperly sizing. It is dangerous, no matter what you call it. :)
On cases that headspace on the mouth, like 9MM, .45, etc, unless you trim a case too short, you cannot create a headspace problem in a firearm where the mechanical headspace is correct.
With revolver rounds that headspace on the rim, there is nothing in normal reloading that you can do to affect headspace, or induce a headspace problem. (Unless you chuck the case in a lathe and thin down the rim ;))
With belted cases that headspace on the belt, you cannot induce artificial headspace. (Unless you chuck it in a lathe and modify the belt ;))
Note: Many people size belted rounds to headspace off the shoulder to increase consistency and case life.
Bottle necked rounds that headspace on the shoulder are really the only ones you need to worry with, as over sizing (bumping the shoulder back too far), can induce excessive "headspace".
Below is an example of excess headspace, the image is a "commercial" .303 British cartridge being fired in a British .303 military chamber that was enlarged for the mud of Flanders in WWI. Please note the words "headspace" and "head clearance", head clearance is the "air space" between the bolt face and the rear of the cartridge case. Notice how the case thins in the base web area due to the excess headspace when fired, military cartridge cases are made thicker in the base web area to better withstand longer military headspace settings.