Where's Case Head When Round Fires?

[Bart B.]

A. Against/touching the bolt face as pulled back by the extractor.

B. Some thousandths inch off the bolt face; the difference between the headspacing limits of chamber and case.

[Nathan]

I’m going with B....well, except when A=B.

[Aguila Blanca]

Rifle or handgun?

Match grade firearm, or mil-surp bargain?

[Bart B.]

Originally Posted by Aguila Blanca

Rifle or handgun?

Match grade firearm, or mil-surp bargain?

Either. Except as long as case head clearance is at least .001" and load is safe to fire.

[jdc606]

My guess, either choice depending on how hard the extractor holds the case rim. In my AR15, can't imagine the firing pin knocking the round free of the extractor and forward into the chamber shoulder. Don't recall how much slop there was between the bolt face and extractor in the control feed Mausers I grew up with. Can't wait to learn the correct answer. Really enjoy reading the knowledge some of you "rocket scientist-type" firearms enthusiasts share.

[Bart B.]

Originally Posted by jdc606

Don't recall how much slop there was between the bolt face and extractor in the control feed Mausers I grew up with. [

Measure the clearance from bolt face to extractor claw in full back positions. Subtract the case rim thickness.

[5whiskey]

I'm going with A. Although the extractor could still give it a couple thousandths of clearance off the bolt face while still holding on.

I believe most chambers offer enough resistance near fully chambered that the round will not continue forward momentum once the bolt has locked. Maybe I'm wrong, and I don't know of any method off hand to test the theory.

[jdc606]

I understand how a person could go about measuring the slop between a Mauser bolt face and the extractor. Problem is our Mausers are long gone/ unavailable. My Dad bought surplus back in the 60's. Most were bought to customize and old Dad liked to polish. Had to replace the extractor on one Mauser bolt that had been polished a bit too much and would no longer properly grip the cases for extraction.
I think the bolt face to extractor space would vary from bolt to bolt but doubtful enough extractor clearance to allow the cartridge shoulder to be pushed all the way to the chamber shoulder in generously sized military chambers.
So my guess is the cartridge goes off before the shoulder stops but still behind the extractor, although not necessarily held tight to the bolt face by the extractor.

[RC20]

CRF

Push Feed? If the head space is excessive enough it will push the case forward and not latch over the rim.

[Reloadron]

This seems to be a continuation of a recent thread where this subject came up. While I decided on A it could likely also be B depending on the rifle and rim thickness, both of which have a tolerance, rim thickness and extractor to breech face (bolt face). I also based my choice of A on what I quoted in the other thread regarding comments by Julian Hatcher. Either way when the bang happens the case can expand forward but has little, very little if any room to travel rearward. Also "some thousandths" is hardly a quantitative measurement.

Ron

[Bart B.]

Does a spring loaded ejector sticking out of the bolt face push a chambered round to its forward limit before the round fires?

If yes, what can stop its forward travel?

If no, why?

[hounddawg]

speaking for my own reloads only I can say with 100% certainty it is exactly where I want it to be judging from the average case life and accuracy I get. Sorry for the non answer but I am kind of a results orientated kind of guy, if it works don't mess with it etc etc

edit I went back and thought about it and voted but I wish there had been a choice C - either A or B depending on a few different factors

[5whiskey]

Does a spring loaded ejector sticking out of the bolt face push a chambered round to its forward limit before the round fires?

I would say generally, I don’t believe it does. I think it can, depending on circumstance, but saving loose chambers I think generally not. Why do I think this? When a round is chambered such that the bolt is in battery, I believe there is likely enough resistance on the round placed by the sidewalls of the chamber, the area around the neck, and possibly the shoulder area, to prevent the ejector from pushing the round off the bolt face. If the ejector does, it still only pushes one side of the head off of the base. Assuming you have a new factory round and not a neck sized reload that closes with slight resistance (we know how the latter will act), I still believe there will be enough contact between the round and chamber to offer sufficient resistance to the ejector.

[jmorris]

Depends, if the case is not loose in the chamber and forced into contact with the chamber and the face of the bolt, it’s against the bolt as much as the primer and firing pin will allow. Really easy to see this if you think of a break open single shot that won’t close without some amount of inertia.

Could also be held off center by an extractor/ejector combination or just be in a somewhat confined “space” and free floating somewhat.

Like a lot of things in life, it depends.

Why, what are you trying to figure out?

[FrankenMauser]

Firing pins and spring-loaded ejectors push cartridges forward until they:
A) Reach the limits of the chamber. - Headspace limit, or bullet contacting throat/lands.
B) Are stopped by the extractor on the rim. (Usually requiring some really sloppy headspace...)


Exception: Long cases in blow-back designs. In some blow-back systems, the bolt can be resting firmly against the case head while the case mouth (or shoulder) is firmly against the corresponding feature of the chamber, without being out of battery.
While possible in a locked-breech system, case dimensions must be carefully controlled. The situation is unusual (or, "rare", if you prefer).


Whether or not the inertia of a firing pin can push a cartridge all the way forward, in controlled-feed, claw-extractor designs, before the mass of the cartridge resisting movement against the firing pin is enough for the primer to initiate and pressure to build, is something that I simply do not know.
So, a case head could, theoretically, be anywhere between the bolt face and headspace limit, at the point that pressure 'locks' it in place and ceases movement.

But, in my opinion, a case head will never be against the bolt face at the moment of initiation, unless the shoulder/mouth is planted firmly against a matching feature in the chamber that will prevent movement.

[briandg]

There must be plenty of space between extractor hook and bolt face so that the inconsistency of rim thickness can be taken care of, and the possible buildup of dirt.

The only reason a case head would be against the bolt face was if the round headspaced solidly on either rim, belt, or shoulder, and I'm really not sure how often factory ammo or even casual reloads will actually fit the gun so well that the things are actually forced against the bolt face.

Stick a shell into a rifle and shake the heck or of it and see if the round rattles?

[std7mag]

Aaahhhhhh....
But Bart said "when the round fires".
So the pressure would push against the brass in all directions, taking up any clearance of chamber to cartridge.
After the pressure subsides, the brass will relax.

[Bart B.]

What am I trying to figure out?

Just curious as to what peoples beliefs are regarding case position in the chamber when the round fires.

Having made a few simple measurements of case fit to chamber in different situations, the facts became clear to me.

[5whiskey]

Having made a few simple measurements of case fit to chamber in different situations, the facts became clear to me.

Feel free to enlighten us. I like learning stuff, and feel like I have a pretty good understanding but confess that I do not know everything. Enquiring minds and all...

[jmorris]

5whiskey this is one way it can go.

https://www.youtube.com/watch?v=w_yN0SESbTA

Won’t always be the case though.

[Bart B.]

Interesting Video by Sinclair, with these exceptions.

* That space between bolt face and case head is correctly termed "head clearance."

* The primer doesn't fire until the case stops moving forward as it's shoulder stops against the chamber shoulder, then the case expands while the primer backs out then reseats at peak pressure as the case stretches back until its head stops against the bolt face.

Easily seen by loading several rounds with incrementally less charge weights in 1% steps. measure case headspace comparing before and after firing. Note the point when primers are pushed out past the bolt face and if case headspace is less after firing.

[Aguila Blanca]

After some cogitation, I arrived at the conclusion expressed by FrankenMauser in post #15.

I'll take "b" for $500.

[RC20]

I got the head space too long in one of my Savage builds and it would indeed not fire.

So yes it can push the case, not sure if there is a line it fires but holds as well.

Most of the time the case head is against the bolt face and then all hell breaks loose when its fired.

[HiBC]

The link will take you to Varmint Al'l Finite Element Analysis study.
Clearance begins with "some"or > 00.00000
There is interference,line to line,and clearance .
Absolute line to line is pretty much theoretical.Real world,there is clearance or interference.

If you have negative head clearance,and are crushing the brass closing the bolt,OK,the head contacts the bolt face.

If you have interference at the extractor hook,the hook can't snap over the rim.

To function properly,the extractor requires some clearance.

Assuming >00.0000 head clearance,between a plunger type ejector and a firing pin strike,the brass has a load to force it forward till it hits a limit. The chamber headspace feature would be that limit. In theory,the total head clearance would be the gap boly face to case head.

Varmint Al was studying chamber finish and friction.What happens after ignition varies somewhat with chamber friction.
Study the Varmint Al page.No need for me to rewrite it.

Varmint Al points out that with common unpolished chamber friction,at chamber pressures below 35,000 psi,the brass likely will not be stretched back to contact the bolt face,so any primer protrusion will remain.
The primer protrusion is strong evidence the case head was off the bolt face.

http://www.varmintal.com/a243z.htm

[Bart B.]

I like this:

Varmint Al points out that with common unpolished chamber friction,at chamber pressures below 35,000 psi,the brass likely will not be stretched back to contact the bolt face,so any primer protrusion will remain.

The primer protrusion is strong evidence the case head was off the bolt face.

Thanks, HiBC.