Where is the case when ignition begins?

[tricrown]

Does the firing pin always push the case of a round with a jumped or low tensioned bullet all the way forward before case expansion begins?

Does the case of a round with a jammed bullet expand forward instead of back to the bolt face? How much neck tension is required to over-ride the push of the firing pin?

How dependent are the answers on the firing pin & its spring?

Thanks,
Bill

I'm wondering if there's a point between .0005-.004 neck tension that allows my jammed bullet round's cases to expand erratically forward and back.

 

[stiller]

Your

first 2 sentences are exactly the way I feel the case reacts. That is the main reason that I think some powders and combinations take lots of tension. If the seating depth is finicky, it is my belief that neck tension must overcome all the forces pushing the case forward. I use lots of tension, maybe too much. My cases are sized such that the inside of the neck is .003 smaller then the bullet. I havent lit the world on fire lately, but that isnt one of the things I have changed since back when I shot pretty good.

stiller

 

[JerrySharrett]

Does the firing pin always push the case of a round with a jumped or low tensioned bullet all the way forward before case expansion begins?

Bill

I'm wondering if there's a point between .0005-.004 neck tension that allows my jammed bullet round's cases to expand erratically forward and back.

This exactly the reason I size cases to where I feel a sight drag on the last 1/4" of travel on the bolt handle knob instead of using a shoulder gage and a fixed setback amount of, say, 0.002".

On most barrels, after you get a pretty good tune, a 0.003" difference in seating depth cam make a barrel move from shooting 0.220" groups to 0.180" groups. (I do mean groups, not aggs)

The case I shoot has a 40 degree shoulder and this helps get a more exact headspace. A case with 30 degrees of shoulder or less acts more like the Morse taper in a lathe tailstock, it can move a long ways between snug and stop.

 

[JSchopper]

Where is the case when ignition begins?

Hopefully in the chamber

 

[alinwa]

Does the firing pin always push the case of a round with a jumped or low tensioned bullet all the way forward before case expansion begins?

The firing pin pushes ALL rounds forward, even those with heavy neck tension and jammed bullet. Furthermore, the cases gener'ly STAY forward.

Does the case of a round with a jammed bullet expand forward instead of back to the bolt face? How much neck tension is required to over-ride the push of the firing pin?

No the case does not expand forward, no amount of neck tension including bullet glued in place can make your shoulders 'blow forward.' The belt on a magnum and the hold-back from the extractor (sometimes) will allow shoulders to blow forward if truly huge clearance is present.

How dependent are the answers on the firing pin & its spring?

I don't understand this question.

Thanks,
Bill

I'm wondering if there's a point between .0005-.004 neck tension that allows my jammed bullet round's cases to expand erratically forward and back

Your cases expand erratically forward and back because of excess headspace. Fix the headspace, eliminate the problem, make good brass. There is no "point between .0005-.004 neck tension" where the problem eliminated. It WILL occur without jammed headspace.

.

Good questions.

I commend you for observing and measuring actual results and finding your problem. Most people just speculate about this stuff.

ONLY jam fit headspace will actually produce good cases although a large number of folks believe that greasing their cases gives them what they want.


Life has taught me that for most people, giving them what they want is more important than actually fixing the root of a problem........ or even defining it!

al

(I'll be lucky if that last statement doesn't get this post wiped off the board since we've got some moderators who'd rather see "agreement" than truth lately. And this statement runs counter to lots of popular opinion....)

 

[Vern]

"The firing pin pushes ALL rounds forward, even those with heavy neck tension and jammed bullet. Furthermore, the cases gener'ly STAY forward"

Dont know if its true or not but a simple experiment would prove or disprove the statment.

Seat a bullet in a case with a dead primer
Measure OAL
Put it in the gun fire dud round
Extract
Remeasure OAL

As long as the lands and grooves dont pull the bullet to a longer length, then if the above statement is true the OAL will decrease because it will as he said the firing pin will push the case forward no matter how much neck tension or glue you use for the bullet.

Other wise if it were full of powder the moment of ignition would begin to push the case back even if it were pushed forward by the firing pin. That is how we get recoil.

If the ignition can not push a case back to where the head is pressed against the bolt face then many who think that the head will be automatically squared by firing are wrong.

Which is it?
I dont know but just sharing some thoughts and a way to verify if the firing pin can overcome neck tension in pushing a case all the way forward on impact.

 

[Charles E]

Life has taught me that for most people, giving them what they want is more important than actually fixing the root of a problem........ or even defining it!
. . .

(I'll be lucky if that last statement doesn't get this post wiped off the board since we've got some moderators who'd rather see "agreement" than truth lately. And this statement runs counter to lots of popular opinion....)

???

Al, your philosophy is pretty good, even if you're sometimes wrong in the shooting-detail dept! Think we can keep it on the board

 

[JerrySharrett]

"The firing pin pushes ALL rounds forward,

Dont know if its true or not but a simple experiment would prove or disprove the statment.

Which is it?
I dont know but just sharing some thoughts and a way to verify if the firing pin can overcome neck tension in pushing a case all the way forward on impact.

I will readily agree that if the loaded round in the chamber is held against forward motion by bullet jam and/or neck tension may me hammered forward by the firing pin. I just fail to believe that small firing pin contact against the soft surface of the primer cup can jam the cartridge body forward when the shoulder of the cartridge is jammed against the chamber shoulder. The physics is just not there!!

 

[tricrown]

Al are you saying that the firing pin will, with anything near acceptable headspace, hit the primer with enough force to drive the bullet further into the lands equal to the amount of headspace. So if you’re shoulder bumping .002” and the bullet is jammed .010. The bullet will be .012” into the lands at ignition. And the shoulder of the case will always be against the shoulder of the chamber?

 

[jo191145]

Well I just ran a super scientific test. NASA was'nt busy anyway

Took my Rossi Coach Gun. Removed the barrels. Cocked the hammer and put a dime on the breech face. Firing pin strike launches a dime roughly 8 feet straight up into the air. I'd say thats more than enough force to jam a bullet into the lands.
Sure a primer is softer than a dime. That would need to be figured in. NASA promises to get back to me.

Some years back when I first read Als theory on jam vs crush on fireforming it made sense to me. Seeing as I shoot a lot of Savages it was a breeze to set my headspace for crush fit on virgin brass.
Besides the fact my brass no longer needs to make that first big stretch to a normal headspace I can't say what the benefits are.
They sure seem to shoot more consistently while fireforming new brass.
Maybe its just in my head. Half of this game is between the ears anyway

 

[Bob Kingsbury]

Vern

In your experiment, how can the cartridge be removed from the chamber,
without affecting its length ? You can't tap it out with a rod and you can't
pull it.

 

[Vern]

I dont seem to have a problem after closing the bolt.
As i stated I never tried it with a bullet and dead primer AND the firing pin.
But Can set my bullets barely in the case mouth and chamber it extract and have a seating depth that does not change when I set up my dies.

 

[alinwa]

Al are you saying that the firing pin will, with anything near acceptable headspace, hit the primer with enough force to drive the bullet further into the lands equal to the amount of headspace. So if you’re shoulder bumping .002” and the bullet is jammed .010. The bullet will be .012” into the lands at ignition. And the shoulder of the case will always be against the shoulder of the chamber?

Yes.

The only thing that will stop the case from moving forward is solid shoulder support.

al

 

[jackie schmidt]

Al

Amazingly simple, huh......jackie

 

[alinwa]

???

Al, your philosophy is pretty good, even if you're sometimes wrong in the shooting-detail dept! Think we can keep it on the board

ohh thank you so veryvery much, is "wrong" an opinion? Or a fact?

there are plenty of folks here who think it's 'wrong' that 'jamming the bullet' won't hold a case back. There are folks here who've written books who state as fact that greasing cases "fixes" the problems inherent to fireforming. And there are plenty folks here who feel that the whole CONCEPT of 'proper' fireforming is over-rated.......that if the shell fits, wear it. "In the real world the wind rules ennyways"

mattera'fact, I been here for what 15-16yrs now??? And have watched a whole lot of my "wrongheaded ideas" become accepted as "fact." I predict that even something as stupid/silly as my fat chambers will find a following, among winners.

Regarding the idea of holding a case back, the late Skip Otto and I fought openly over this one for several years.... but I gotta' hand it to The Skipper, HE TRIED STUFF! After I published the results of my experiments he went out and TRIED IT. (But only after spending an inordinate amount of time "truing caseheads" might I add.... and posting his results.)

I miss Skip. And FLM...... and now Davey Dohrmann........Inveterate Itchscratchers all, concerned with facts. These men all made the world a better place.

dang



I'm actually kinda' curious as to where I've been "wrong" because I'd sure like the record to be set straight...... it's a public learning forum. I'd hate to be responsible for the promulgation of myth and poor observation as "fact."

I thought I'd cleaned up all my messes.

al

 

[stiller]

I dont look

at the other forums on this much, but this is the first I heard of Dave Dohrman. This news has made a long week even longer. Dave was one of the nicest guys ever in benchrest, in life for that matter. I have all I can do to fight back the tears. I guess all of our times are coming, but I sure am getting tired of being reminded of it.

 

[alinwa]

at the other forums on this much, but this is the first I heard of Dave Dohrman. This news has made a long week even longer. Dave was one of the nicest guys ever in benchrest, in life for that matter. I have all I can do to fight back the tears. I guess all of our times are coming, but I sure am getting tired of being reminded of it.

You said it....

Thanks Jerry

al

 

[David Halblom]

if only the primer, et al, were that soft

I will readily agree that if the loaded round in the chamber is held against forward motion by bullet jam and/or neck tension may me hammered forward by the firing pin. I just fail to believe that small firing pin contact against the soft surface of the primer cup can jam the cartridge body forward when the shoulder of the cartridge is jammed against the chamber shoulder. The physics is just not there!!

then it would not be an issue. If you were to put a delrin plug in a cartridge rather than a primer, and seat a bullet, long (jammed), l'll bet a steak dinner you would be VERY surprised how far forward the case will move. My bet is way beyond the .001 of the shoulder bump. Firing pins have way more force than we give credence to.

 

[alinwa]

then it would not be an issue. If you were to put a delrin plug in a cartridge rather than a primer, and seat a bullet, long (jammed), l'll bet a steak dinner you would be VERY surprised how far forward the case will move. My bet is way beyond the .001 of the shoulder bump. Firing pins have way more force than we give credence to.

for really shallow shoulders like the 30-06 and other old cartridge designs (15-18degrees) you can easily move the shoulder by just slapping the bolt too hard. Checking headspace on a 30-06 using the tape method can be frustrating as you can move the shoulder by closing the bolt. For this reason alone several machine gun designs will not function using the 30-06 cartridge.

For any who want to check whether or not the bullet can be "held in place with neck tension) all's ya' got's to do is set your loaded round on the table and drop a 20lb cinderblock on the nose........ there, did she hold??

OK, this isn't egg'ZACKLY how I tested these myownself but hey.... I don't want to make it too complicated. I want folks to TEST stuff!

Leave the speculation to Goldman Sachs...........

LOL


BTW David...... remember the delrin snap caps of yore??? Seemed a neato idea at the time, just prime a few cases with the delrin plugs and have at it. The 45ACP headspaces on the casemouth, the -06 on the narrow, shallow shoulder......the .308 (20* shoulder) was a popular choice of course with the stand on your hind feet guys......

Kinda' hard to find the buggers these days. Din't work out so veryvery good eh!


Ohhh, and BTW, for those few experimental sorts who may actually TEST this whole notion. Be warned, primers can be HARD to kill! Don't just soak 'em overnite in your brew of choice and yerk the trigger over the kitchen table...... WEAR ear plugs and eye plugs and point in a safe direction etc.... even if the first few don't do nuttin'.........



al

 

[mturner]

The 30-30

This is one reason I'm so fond of the 30-30. I face all my rims to the same thickness, and set the rim headspace to .0005" or less. I never bump the shoulder, so maintaining as close to zero forward case movement as possible is a breeze.