Sunday, December 6, 2009

Headspace

CZ from B&BS armoury

Apologies to all but I've badly neglected the gun side of things; so here's some headspace technics from Tom at Boomers - no mean info for those, like me, who enjoy milsurp rifles and more importantly shooting them. Thanks mate; you'll be glad to know I made the effort to get my Lee's headspace checked by a 'smith...

A lot of people think they understand headspace, but they really don't. Even a lot of gunsmiths, reloaders, and otherwise intelligent people. The Parson asked me if I might be interested in writing a guest bit and I thought this is something that would be worth addressing as there are so many mythical ideas out there. Static headspacing is pretty simple, either the cartridge chambers within .001-3" of chamber print for cartridge or it doesn't. There are many problems, though, with this simplistic view of headspace. I shall attempt to address them in detail.

To simplify things, we shall assume that all cartridges are exactly the proper over-all length, which is of course not true with any factory ammo. We shall also assume that the chamber throating has been properly done. Also a falsehood in 99.9% of factory barrels.

First off, SAAMI specs are sloppy. Going by field gauges and go and no-go gauges will keep your case from rupturing, which is a good thing, but it isn't much of a path to accuracy or precision. Gauges generally accept .005-.006", I don't. You will see why this is important shortly.

Headspace is not a static thing in the firing of any kind of metallic cartridge firearm for a number of reasons. First off, there are the variable of action strength, design, and metallurgy. Break action guns are the springiest and falling block and interrupted thread artillery style breeches (almost never seen on small arms any more) are the stoutest. Therefore starting with perfect or near perfect headspace in a static sense in break action guns is especially critical. The weaker the action the more critical it is to get things right. Blowback semi-autos, delayed or not, also are sensitive not only in accuracy but in probability of misfire if headspacing is not correct.

When you have chambered a round in your firearm with a proper OAL cartridge and SAAMI acceptable static headspace all is apparently well. What goes un-noticed is what actually happens when you press the trigger and fire the round, as it's not directly observable under normal conditions in most types of firearms. When the hammer drops/striker moves forward and the primer is whacked, you are hammering SHARPLY AND VERY HARD on the primer if all is well. The denting of the primer cup absorbs much, but not all of this energy. You are whacking a tube of thin, soft, brass (varying in those properties somewhat depending on how it was annealed and if it has work hardened or not) and more gives than just the dent in the primer. We assumed proper OAL and chamber but keep in mind for reliability: If the chamber is wrong or the OAL is wrong, or both, you may get a misfire beause the primer strike mostly punted the cartridge forward in the chamber and you got a light primer strike resultant of that.

Break open guns have a great virtue for examining the results of this because as my amigo Mike Bellm of Bellm TCs has said many a time, "You CAN DIRECTLY not only SEE what happens, BUT you can also measure the results." He and I have discussed this many a time regarding accuracy and misfires, He is lucky enough to be working with BPI/CVA/Bergara Barrels on product designs and manufacturing methods and recently was over in Spain working with the Bergara Barrel Factory. In a recent email from Mike, he mentioned:

This today from the head tech at BPI/CVA/Bergara that I work with regularly and who took me to the Bergara plant in Spain the first of last month:

"I don't remember if I mentioned it to you or not but I dropped some loaded .35 Whelen rounds in a barrel and measured from the breech to the ctg base.

Average about .005". This was very consistent through this partial box. I
then dropped in a few misfired rounds that had been hit by the firing pin.
The average measurement went from .005" to .014"! Proof enough for me to
believe that the blow of the firing pin is crushing the shoulder."

..... and I have to add, all the more reason for YOU to be able to take the measurements and find these things for yourself, then make your own corrections as necessary.

That is a total collapse of .009" at the shoulder!


Rather interesting, is that not? Belts on belted magnums provide some insurance against such behavior, provided the belt is properly placed on the cartridge and the chamber is properly cut for the belt, which is not always true. I've gotten
A-Square brass where the belt was misplaced significantly. I'm naming them directly here because if you Google their brass prices for safari rifle cartridge brass alone, not cartridges, you can see why it was vexing when they refused to talk to me about possibly replacing it with dimensionally correct brass. Didn't answer the phone messages or emails. Belts .008-.011 short of appropriate forward bearing surface are not acceptable and are of no use but scrap/example of shoddy product. I know this is a Christian page but some things aren't very forgivable if you wish to remain in business. Anyway, if the belt is wrong or the chamber isn't cut properly for the belt, your headspace will be variable dynamically even with a belted magnum designed to prevent case movement and assure proper headpsace because it will be headspacing via projectile on the chamber throat/barrel leade and/or case mouth and/or shoulder. Rimmed cartridge movement is primarily determined by the strength of the rim, provided the chamber and OAL are correct and the counterbore for the rim is correct.

So, you see why .001-.002' of headspace is a good idea for consistent shooting now? Dynamically, your acceptable static headpace very well may not be acceptable.

You can test your own firearms by loading empty cartridges with deactivated primers to correct OAL and "firing them". Then you'll know how much variation there is in the brass you are using/whether or not there are chamber issues. Then fix what needs to be fixed, if you find much variation. You'll make misfires very unlikely and gain accuracy.

Remember, mi amigos, all firearms may be interesting, but only accurate firearms are particularly interesting for anything but hanging on the wall and making loud noises.

Happy Shooting.


And a blessed Advent,

LSP

11 comments:

Rick Kratzke said...

Pretty interesting what I did understand, it is good to learn something new every day. I am going to read it a couple more times to try and understand the rest.

LSP said...

Thanks Rick - Tom's post is certainly a teaching thing for me too!

tom said...

UPDATE: Slightly email chastised by Mike for even mentioning chamber prints and cart specs as the thing is what you really need to do is measure what your true headspace is when the cartridge is going bang and such. The reason people get confused about headspace is by thinking gauges are a way to measure it anything more than nominally. You want minimum headspace in reality by testing and measuring things like shoulder collapse.

or as mike said:

Screw the prints and specs entirely..... ammo and chambers do not equal reality. Work with the reality in hand and screw the prints and/or gauges.

That's the point.

You did do well with the fact that shoulders collapse, though. ;-)


Pardon me for commenting on my own post. Dial indicators and/or feeler gauges, depending on action design = good. Headspace gauges = bad. Measure the reality not the theoretical

tom said...

Thinking about it, this might help people wrap there head around what I was trying to get across in simplistic terms that can't be misunderstood.

When a metallic cartridge is fired:

Primer strike causing some forward movement and deformation of the cartridge case.

As the charge burns, the case spends some time sorta in mid air so to speak, due to a temporary forward crushing of the case.

Then the case lands against the breech face of the action and gas pressure expands the case to the full dimensions of the chamber.

Proper headspacing keeps the ping ponging around in mid-air, so to speak, to a minimum. And the only way you get that is my measuring the realities not using go/no-go gauges. By dynamic testing, as the feller in Spain did.

The less ping ponging around, the better you are in accuracy and preventing misfires.

Hope that makes sense to y'all.

Some people think that's snake oil but that's the actual mechanics of the beast just as sure as whale oil is a better patch lube than mink tallow. Not that it's sold anymore, but they found it to be true back when whaling was more popular. Just like headspace, they found it out not by theorizing and using static tests, they dynamically tested things at the range and in the lab.

The proof is in the proofing house as well as in the pudding.

LSP said...

Thanks Tom - helpful.

tom said...

Mike just emailed me a chiming in on my update that cases mostly expand and then slide back or do them somewhat simultaneously, depending on chamber finish. He's experimented with that a lot. Different chamber finishes having a significant influence on the proceedings as to the cartridge moving around in the firing process once the case starts to expand.

"Take the extractor out of a barrel for a bottle neck case. Start you loads low, and you will find then stay FORWARD in the chamber until you reach about 40,000, at which point things begin to move. Depends very largely on the surface finish inside the chamber."

Tom writes a guest tech article and learns more things from his friend ;-)

Best way to find out you are mostly right in your answer is by opening your mouth and asking your gurus if you said things properly...

Kinda funny...beating up my own web post. I usually do that to other people...

LSP said...

Humility is great virtue... tempted to do a bit on the blank lock...

Bob Qat said...

CZ 52?

LSP said...

not sure.

tom said...

VZ-52 (Sometimes referred to as "Che Rifle")

Original and better chambering of 7.62x45 before Warsaw Pact politics made them rechamber/rebarrel most of them in 7.62x39

Semi-Rare.

Used in a lot of Warsaw Pact Client State Militaries and Insurgencies, PRIMARILY in Africa and Latin America, although some went to the Mid-East and even the IRA. Of course the Czechs used it in their own Military and police forces.

Battle rifle not assault rifle. Essentially it's a amalgamation of various semi-auto rifle designs the Czechs liked, Primarily based off of German stuff they produced for the Nazis during the war and a number of ideas stolen from the M-1 (Garand) U.S. Rifle of .30 Caliber.

Very well made and accurate rifles. Ammunition is troublesome unless you make cases yourself from something else, of which there are a number of ways to do, because the origianl ammo is both CORROSIVE and REALLY OLD. I've shot some CZ Mil Surp surplus in it in the past but then you have to clean the gun (not really bad as they take down about as easily as a Berretta 92FS) and the old ammo has a tendency to hang fire because of the powder choice they made way back when and how it tends to age. Boxes of original ammo I have left for it I'm just keeping as collector's items, shoot modern non-corrosive in it when feel like shooting it.

Sometimes you see them at gun shows.

Neat rifles if you like Mil-Surps.

Cheers,
Boomer Lad

tom said...

CZ-52 is a semi-auto pistol, FWIW.

People confuse them all the time on the web. Vexing people ;-)