Bullet Length in Relation to OAL
- Thread starter scatterbrain
- Start date
that is way too easy and informative
edit - after a careful re reading of Tubb's paper apparently BartB is correct, the firing pin does transfer sufficient energy to set back a case shoulder.
https://www.davidtubb.com/index.php?route=account/download/free&download_id=28
in section 5 FIRING PIN IMPACT STUDIES
guess this is my "learned something new today" factoid
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Bart B.,
That is what I was referring to with this..
While all this may be true, it all happens AFTER you have loaded ammunition. And so has no bearing on the overall loaded length of the round. SO, as far as I can see, the whole matter of how the round headspaces, and what happens when the firing pin falls is something after assembling the round and is a separate matter from differences in COL due to bullet seating and variances in bullet length.
Sure, its important stuff, but I don't see how it applies to the OP's question.
No it doesn't , not even close I can't believe you even said it . What you say may be true with factory ammo but we're not in the factory ammo sub forum . I can change every single one of those things individually as I load the cartridge . The case head space ( head to datum measurement ) in not fixed . It is the distance I choose it to be for each case . Although not recommended every case can be different then the next to as much as .014 or so depending on the die adjustment . As is where I seat the bullet in relation to the baring surface and the case shoulder datum . Also my test just showed that in at least one example the shoulder was set back by the firing pin strike . Does that mean one should account for that and size cases accordingly ?
In fact before I started using the Redding competition shell holders my head to datum measurements varied as much as .005 from case to case do to press deflection and case spring back . in that instance If I'm measuring head to ogive to get my .005 off the lands . My head to datum variance of .005 will cause some bullets to be touching while others are .005 off or .005 to .010 off . I'm not one that thinks just a few thousandths variance in seating depth is a big deal but based on what the OP appeared to be trying to do . Which was make everything as close to exact as possible . I can see how this info could be something one "may" consider .
My AR-10 has a headspace .001 short of a field gauge . This means factory ammo at times can have as much as .012 head clearance in that chamber . Trust me I know it is VERY hard on factory brass . When I bump the shoulders back .003 from fire formed in my AR-10 . Those cases will not chamber in any of my other 308's yet the AR-10 runs just fine and is a lot less hard on that brass .
I believe the OP and I both agree and has been my point through examples in most of my posts . The question was asked , why is that line of discussion even in this thread ? I simply explained it as I understood the "theories" that were being put forward , I was not indorsing them just trying to help explain them .
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In MarkCO's response to my statement that (rimless bottleneck) case shoulders stop against chamber shoulders from firing pin impact:
Then what do you think the case stops against?
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I'm asking for your opinion on what stops the case.
If you don't have the wherewithal to tell me, so be it.
This grossly exaggerated drawing of difference in case head-to-shoulder length may make it more obvious why the shoulder position controls the bullet jump when bullets are seated the same depth into the case mouth.
The firing pin study is interesting, but it still doesn't account for the primer's pressure, which we know makes some contribution, as John C. Garand's first semi-auto rifle design was primer-actuated. I have a way to measure it, but it requires a bit less snow cover be on the ground so it will need to wait for the weather to improve.
The firing pin study is interesting, but it still doesn't account for the primer's pressure, which we know makes some contribution, as John C. Garand's first semi-auto rifle design was primer-actuated. I have a way to measure it, but it requires a bit less snow cover be on the ground so it will need to wait for the weather to improve.
Attachments
This gives me the impression we are talking about different things.
What I am talking about, what I understood the OP to be asking about is differences in the cartridge over all length, due to differences in the length of the bullet used.
I am NOT talking about what happens when you chamber and fire the round, only about things that happen when you are seating the bullet when assembling the round.
Where the case shoulder is, is irrelevant when seating the bullet. Your seating die does not (and should not) touch the case shoulder. Because there is no contact with the die body, the exact position of the case shoulder doesn't matter when seating the bullet and then measuring the over all loaded length, from cartridge base to bullet tip.
It does Unclenick, but the drawing is missing a bolt face and extractor. Neither drawing is even possible as the casehead (at least a portion thereof, in some manner) is not within the chamber.
In between the extractor forcibly engaged to the head side of the extractor groove to shoulder dimension and the bolt face to shoulder dimension, is where whatever amount of possible extra headspace occurs. How large that distance is will determine, in part where the "extra headspace" is allocated...ie at the bolt face and or the shoulder. In this paragraph, extra headspace just means the amount over zero, not necessarily dangerous, like excessive headspace.
In between the extractor forcibly engaged to the head side of the extractor groove to shoulder dimension and the bolt face to shoulder dimension, is where whatever amount of possible extra headspace occurs. How large that distance is will determine, in part where the "extra headspace" is allocated...ie at the bolt face and or the shoulder. In this paragraph, extra headspace just means the amount over zero, not necessarily dangerous, like excessive headspace.
MarkCO,
It is not meant to be realistically proportioned. That would make differences too small to see clearly in a drawing. I labeled it as "grossly exaggerated" exactly for this reason. The idea is to show the principle Bart brought up, which is that the difference in shoulder-to-ogive length is what sets the bullet jump at firing.
For the OP's measurents, measuring from the bullet's ogive at its throat intercept point to the point on its ogive where the seater stem's ram makes contact will provide information about how much real variation the irregularity in bullet shape will cause.
It is not meant to be realistically proportioned. That would make differences too small to see clearly in a drawing. I labeled it as "grossly exaggerated" exactly for this reason. The idea is to show the principle Bart brought up, which is that the difference in shoulder-to-ogive length is what sets the bullet jump at firing.
For the OP's measurents, measuring from the bullet's ogive at its throat intercept point to the point on its ogive where the seater stem's ram makes contact will provide information about how much real variation the irregularity in bullet shape will cause.
scatterbrain
New member
If you neck size how much would your firing pin push the case forward? The cases have been annealed and ALWAYS measure to the same length after firing. This may be a little bit off OP, but.
It's probably just going to be a thousandth or so. The push forward will be equal to the head clearance that remains after spring-back. But if your cases all stay the same size as each other, then that will be consistent and measuring from the head to the throat contact point on the seated bullet's ogive will be as good an indication of consistency as a separate measurement between that point and the shoulder would be. If you use a bullet comparator insert like the Sinclair that meets that ogive-to-throat location, it will tell you if the seater is touching an equally consistent part of the ogive or not.
If fire formed and assuming the shoulder is fully blown out . The case should not move forward in the chamber . Keeping in mind in many firearms the case in reality will not be pushed forward bcause the ejector has enough force to keep the cartridge fully forward leaving head clearance if any rather then shoulder clearance that would allow the case to move forward.
The case will be driven forward hard against the chamber shoulder.
The fired case will have a few ten-thousandths inch more head to shoulder dimension.
Subsequent neck only resizings will eventually bind the bolt closings.
Then the case needs full length sizing setting the shoulder back a few thousandths.
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Of course not.