I ask the following with some trepidation .....................

[Macropod]

If the rims on your cases have been damaged from pulling the cases out of the die and you measured after this, then once fired they would likely have set back hard against the bolt face. This would give the impression of reduced measurement.
Not as likely as the above scenario but possible nonetheless.

 

[BoydAllen]

Hi boydallen

Thanks for this, but can't I can't grasp the analogy!

I fail to understand that when I have a case that measures 2.005 inserted into a chamber that is longer and then fired, the case then is shorter post firing.

You say "minimal longitudinal clearance at it's shoulder" but that's not the case (pardon the pun), the 7.62 chamber is .013 longer and therefore the clearance will be significant?

Did you do what I suggested or just think about it?

 

[snert]

Hi Cam,

Brass is a solid until it is fired, when it momentarily becomes elastic. It swells to fit the WALLS of the chamber, sealing it so the gasses do not go backwards. When the walls of the chamber are of a generous size as they are in your chamber, the material (brass) swells more to reach the walls. As it swells it draws the neck backward a bit. It is counter-intuitive, but that is what happens. Then in a micro-instant, the brass becomes a solid again, and changes shape a bit, but does not return to its original state. (If it did we would not need to size the case). Over a few firings it is likely that you will find the neck lengthenig, as the brass also "flows", and the material moves forward.

See my PM for more

 

[Campbell]

Did you do what I suggested or just think about it?

Hi BA

Yes I tried it, but being a dumbass accountant the exercise was lost on me! I will persist though until I understand what's going on.

 

[Campbell]

Hi Cam,

Brass is a solid until it is fired, when it momentarily becomes elastic. It swells to fit the WALLS of the chamber, sealing it so the gasses do not go backwards. When the walls of the chamber are of a generous size as they are in your chamber, the material (brass) swells more to reach the walls. As it swells it draws the neck backward a bit. It is counter-intuitive, but that is what happens. Then in a micro-instant, the brass becomes a solid again, and changes shape a bit, but does not return to its original state. (If it did we would not need to size the case). Over a few firings it is likely that you will find the neck lengthenig, as the brass also "flows", and the material moves forward.

See my PM for more

Hi snert

I understand everything you have written (I have not looked at your PM yet) however if my chamber is .013 longer (I understand that the SAAMI spec is the same for the diameter of the 2 cartridges) I am having difficulty understanding why my cases case are not being fire formed into something longer. This is the only bit I don't understand, and I am missing something.

I am speaking with an engineer/gunsmith who often endures my quizzical nature!!!!

Thanks again

 

[Campbell]

Did you do what I suggested or just think about it?

I have been walking around the house imitating a Shaolin Monk

GOT IT! (I think)

Am I correct in thinking: The finger tips are analogous to neck of the case, and the heels of the palms are analogous to back of the shoulder of the case?

Cam

 

[dkhunt14]

If you take a long skinny balloon and start blowing it up. As the balloon gets bigger, it gets shorter. That is just like your case, as it expands to fill the chamber, it is pulling brass from the shoulder neck area and it gets shorter. If you would jam a bullet in the lands and fire, the case wouldn't move away from the bolt face and would fireform to a slightly longer length. It would get shorter but not quite as much. Matt

 

[snert]

I have been walking around the house imitating a Shaolin Monk

GOT IT! (I think)

Am I correct in thinking: The finger tips are analogous to neck of the case, and the heels of the palms are analogous to back of the shoulder of the case?

Cam

yep!

 

[snert]

Also, keep in mind that SAAMI specs are written with min/max tolerances. So as long as the chamber is smaller than the big SAAMI spec, and bigger than the small spec it is "good". NATO chambers tend to run toward big, so they don't hang up when needed most.
DIES have to be cut to squeeze a cold piece of fired brass of unknown brittleness (not a molten piece of brass forming under fire in a chamber) down to remain UNDER the minimum SAAMI spec after spring back so it would fit into the theoretical minimum SAAMI chamber.
So if you have a big chamber, you are sizing the brass way down in that die. Hence the ability to adjust the die! To some degree you can size the case "less" by backing the die out. Just make sure it always has contact with the shellholder when the ram is all the way up. Backing it out any more and you incur other issues for discussion in another thread.

Best advice I can give you is buy 100 new Lapua, use your FC to adjust the die so it minimally sizes and still fits your chamber, follow the details of my PM with those Lapua, and sell the FC for scrap. They already have at least 4 firings and FC is soft, and yours too short. Cut your losses and use good brass.

 

[snert]

yep!

and nope...yes, the neck, and the heel is the rim. Think OAL of case. Now allow the middle to swell outward into a bigger diameter. Dont move the case head. The fingers (neck) move back, shorter.

 

[BoydAllen]

I should have added, "keeping the palms parallel".

 

[Campbell]

I should have added, "keeping the palms parallel".

Hi BA

Your analogy is even more clearer to me now! Many thanks

Cam

 

[Campbell]

OK, many, many sincere thanks for all of your help.

I understand the principle of the finite amount of brass "flowing" to where it needs to fill voids, and will have to be accomadated from some area(s) of a case. However, the bit that keeps banging into my thick-head is:-

a) Surely, the base-to-shoulder measurement on my fired case must be longer than a Win 308 (given that the 7.62 chamber is .013 longer?

b) If a) is correct, then surely the shrinkage must be in the parallel neck of the case?

 

[BoydAllen]

Where did you get the idea that a 7.62 x 51 chamber is .013 longer? What is your source for this? They are the same cartridge. One is the American designation and the other the European. As far as I know the dimensions are identical. It seem to me that rather than research this you have made an assumption.

 

[dkhunt14]

Where did you get the idea that a 7.62 x 51 chamber is .013 longer? What is your source for this? They are the same cartridge. One is the American designation and the other the European. As far as I know the dimensions are identical. It seem to me that rather than research this you have made an assumption.

I believe he got it from post 10. Matt

 

[wholman]

do you have the tools to measure the shoulder and ogive of bullet.
They will help you to set up dies .

 

[Mr Underbridge]

Behold my mighty artistic skills!

 

[Mr Underbridge]

The arrow represents a "boom" after you pulled the rigger

 

[Campbell]

Where did you get the idea that a 7.62 x 51 chamber is .013 longer? What is your source for this? They are the same cartridge. One is the American designation and the other the European. As far as I know the dimensions are identical. It seem to me that rather than research this you have made an assumption.

Hi BA

I got the data from: -

http://www.303british.com/id36.html

Many thanks

Cam

 

[Ggmac]

Too many mistakes in that article . Not enough good info . Did he full length resize his fal or midland cases . One case doesn't fit another .
Go and no go field gauges are a max . Look at chamber / reamer prints , the chamber / reamer will be same but for throat / leade .
And the midland was a bastard rifle using us Springfield bolts and a Mauser barrel . Bought by the Gibbs rifle company ( formerly Navy Arms ) here in the good old USA .
Allowable NATO specs will always be larger , ALLAWABLE is the key word .