Chamber question

[*MALICE*]

Ok, maybe a silly question......

Let's say you have a barrel chambered for "X" caliber (rimless), and let's say that the chamber is the minimum dimensions (go gauge JUST fits). Let's also say that the "gap space" (for this question we'll use the distance from the breech face to the bolt face as "gap space", i don't want to get in a debate on the definition of "headspace" ) is .100" .

Now, if you were to take an empty casing for caliber "X", resize it with your full length resizing die, trim to length, then drop it in the chamber, would the head protrude .100" from the breech, or will it drop in farther than that?

 

[guntech]

Ok, maybe a silly question......

Let's say you have a barrel chambered for "X" caliber (rimless), and let's say that the chamber is the minimum dimensions (go gauge JUST fits). Let's also say that the "gap space" (for this question we'll use the distance from the breech face to the bolt face as "gap space", i don't want to get in a debate on the definition of "headspace" ) is .100" .

Now, if you were to take an empty casing for caliber "X", resize it with your full length resizing die, trim to length, then drop it in the chamber, would the head protrude .100" from the breech, or will it drop in farther than that?

It will probably drop in farther because most new cases are undersize to start with.

If it had been correctly fireformed in this chamber to start with, you would still have to set your sizing die correctly or you may push the shoulder back and create a case a bit too short... Sizing dies can not just be screwed down to hit the shellholder - they need to be set for the individual rifle so the shoulder of the case is not pushed back... a fully sized empty case should be felt as the bolt closes on it.

 

[*MALICE*]

ok, so, if it was a new case and it was undersize (ie. the case protruded less than the .100") when the gun goes bang, will the case fill out the chamber, or would there be an issue with blowback (assuming that the rifles headspacing is correct)? Since there is some play, obviously, between "go" and "no-go", one would figure that if you had a chamber that was close to "no-go" but still within spec, and your ammo was sized for "go", that there will be some play, meaning that the case would HAVE to expand to fit the chamber.
My question isn't so much in regards to top-notch accuracy as it is to is this a somewhat "normal" condition.

 

[guntech]

ok, so, if it was a new case and it was undersize (ie. the case protruded less than the .100") when the gun goes bang, will the case fill out the chamber, or would there be an issue with blowback (assuming that the rifles headspacing is correct)? Since there is some play, obviously, between "go" and "no-go", one would figure that if you had a chamber that was close to "no-go" but still within spec, and your ammo was sized for "go", that there will be some play, meaning that the case would HAVE to expand to fit the chamber.
My question isn't so much in regards to top-notch accuracy as it is to is this a somewhat "normal" condition.

When a cartridge is fired the firing pin drives the cartridge as far forward as it can go (the shoulder stops it on a rimless case). Then as the pressure builds the brass will stretch at the rear of the case and will be stopped by the bolt face. If the pressure is not high enough for the brass to stretch, often the primer will end up protruding slightly.

Chambers have minimum and maximum tolerances.
Ammunition has minimum and maximum tolerances.
A maximum tolerance cartridge has to be able to fit in a minimum chamber.
A minimum tolerance cartridge will have quite a bit of clearance in a maximum tolerance chamber.
This is all "normal".

 

[*MALICE*]

super dooper! thanks a lot, guntech.

 

[gunster-old]

Hi Malice
To end up with cases that have the correct dimensions for your chamber may I suggest the following;

(a) Load the new case with the bullet seated out to touch the lands of the rifling. This will hold the base of the case against the bolt face and prevent protruding primers as mentioned by GunTech. If you have protruding primers you have excessive headspace.

(b) Use a loading manual and choose a load between the "Starting" load and the "Maximum" load. This will give you enough pressure to fully expand the shoulder of your new case.

Regards;

 

[guntech]

Hi Malice
To end up with cases that have the correct dimensions for your chamber may I suggest the following;

(a) Load the new case with the bullet seated out to touch the lands of the rifling. This will hold the base of the case against the bolt face and prevent protruding primers as mentioned by GunTech. If you have protruding primers you have excessive headspace.

(b) Use a loading manual and choose a load between the "Starting" load and the "Maximum" load. This will give you enough pressure to fully expand the shoulder of your new case.

Regards;

It is a extremely common misconception that a bullet jammed into the lands will hold a case against the boltface for fire forming.

In fact a bullet jammed into the rifling will not hold the case against the bolt face when the firing pin hits the primer with 24 pounds of force. The bullet will be driven into the case and the case will move forward until the shoulder contacts the chamber.

If you want to correctly fireform, you need to create a "false" shoulder by expanding the neck diameter and then only sizing enough so you can feel the bolt close quite firmly. That will hold the case against the bolt face.

 

[Calum]

Good info guntech, interesting stuff.

 

[gunster-old]

It is a extremely common misconception that a bullet jammed into the lands will hold a case against the boltface for fire forming.

In fact a bullet jammed into the rifling will not hold the case against the bolt face when the firing pin hits the primer with 24 pounds of force. The bullet will be driven into the case and the case will move forward until the shoulder contacts the chamber.

If you want to correctly fireform, you need to create a "false" shoulder by expanding the neck diameter and then only sizing enough so you can feel the bolt close quite firmly. That will hold the case against the bolt face.

Dennis, you da man. Forming a false shoulder as you describe is the best method used to fire form cases. A bullet into the lands also works as the extractor holds the case against the bolt face during ignition. It has worked well for me for many hundreds of cases but it is not as precise as the false shoulder method.

 

[bigbull]

Creating a false shoulder is a better way of fireforming but seating the bullet into the lands also works unless you have a loose fitting bullet sometimes this can happen with thin necks. I have fireformed many cases by seating the bullet into the lands. The one place where forming a false shoulder really works well is when fireforming into a different caliber eg. when you form Ackley calibers.
bigbull

 

[guntech]

Creating a false shoulder is a better way of fireforming but seating the bullet into the lands also works unless you have a loose fitting bullet sometimes this can happen with thin necks. I have fireformed many cases by seating the bullet into the lands. The one place where forming a false shoulder really works well is when fireforming into a different caliber eg. when you form Ackley calibers.
bigbull

Try seating the bullet out on an undersize primed case with no powder and measure the length... dry fire it and measure the length. I'll bet the bullet got driven in the case...

You can successfully fireform by a lot of methods... successful if you don't care about the brass stretching.

The only guaranteed way to prevent the brass from stretching is by creating a false shoulder. It will produce a stronger fire formed case.

 

[tiriaq]

A properly reamed Ackley chamber does not require a false shoulder. The factory case should headspace firmly at the neck/shoulder junction. There are wildcats that do require the shoulder to be blown foreward, and creating a false shoulder works for these.
I'm surprised that no one has mentionned lubricating the case to prevent it from adhering to the chamber walls, thereby facilitating fireforming with reduced case stretching. It also dramatically increases backthrust on the locking system, and is therefore a questionable technique.