Bullet Length in Relation to OAL

[Unclenick]

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.

[44 AMP]

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 .

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.

[MarkCO]

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.

[Unclenick]

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.

[scatterbrain]

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.

[Unclenick]

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.

[Metal god]

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.

[Bart B.]

Originally Posted by scatterbrain

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.

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.

[Bart B.]

Delete

[MarkCO]

Originally Posted by Bart B.

Are you calling the distance from the bolt face to case head "head space?"

Of course not.

[stagpanther]

A fascinating discussion and thanks to all contributors--this is one of those instances where a Moving digram might help, at least for people like me who have trouble following exactly what's going on.

Somebody brought up AR 10 and since I've done what are referred to as high-pressure AR 10 builds--which is not to imply the routine running of charges beyond max pressure ratings for the cartridge--I thought that I might make an observation that may or may not have a bearing on the conversation. AR's don't have a firing pin spring per sae, it's more of a kinetic energy transfer from the bolt carrier to the free-floating firing pin.

I've noticed that brass in cartridges like the 6 and 6.5 creedmoors can get over-worked quickly from having to constantly set the shoulder back and then all kinds of problems can derive from that. When I switched to JP's high pressure bolts--and in particular their lighter weight and narrower-diameter firing pin--those issues were significantly alleviated.

Here is their description.

[Metal god]

This gives me the impression we are talking about different things.

There may be some truth to that

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

I'll need to go back and look or maybe he can clarify but I think Unclenick was talking about a seating die or Mic the bases your seating depth off a shoulder measurement ??? I think in general I agree with you but this thread has dove into some very advanced reloading techniques and theories that many may not know about or even think are relevant .

Moving diagram might help, at least for people like me who have trouble following exactly what's going on.

That would speak volumes I think .

When I looked at UN's diagram I felt it was good but needs a third diagram or all the diagrams with a cut out of the neck showing the depth of the bullet . I think if the exaggerated diagrams each had the bullets seated to the same length from head to bullet ogive which would be illustrating both having the same COAL . With a cut out of the neck and them both being fully forward in the chamber . You would see clearly how case headspace ( distance from case head to datum on the shoulder ) will effect where the bullet is in relation to the lands of the bore when the cartridge if fully forward in the chamber

In that diagram you would see the cartridge with the exaggerated head clearance would have absolutely jammed the bullet DEEP into the rifling while the properly sized cartridge would keep the bullet off the lands . The neck cut away would just show the extreme seating depth difference you'd have if both cartridges had the same COAL but one had a significantly shorter head to datum measurement .

I understand Unclenick's diagram is showing how seating depth based off the case shoulder to bullet ogive will not effect where the bullet is in relation to the rifling when the case is fully forward regardless of head clearance . However most reloaders IMHO base there COAL off the head/base to bullet tip or ogive . Therefore I think the above method/diagram may be easier to follow for the rest of us

Alright , now we all should be properly confused lol .

[Metal god]

Added to post above so to not double up posts

[stagpanther]

The part my feeble mind is grappling with is--if the case shoulder is set back by the force of the firing pin--then why would you subsequently need to bump the shoulder back upon resizing? Is there a "rebound" effect in the growth of the case body between the head and the shoulder back in the direction of the bolt breech face upon ignition?

I mentioned the JP firing pin because I have used them and they have definitely made a difference in my high-pressure AR-10's, though I can't expertly define why, and what I have observed is in line with what JP claims.

I shoot lots of belted magnums, and due to their scarcity I've sniveled some used cases from hunters who are not reloaders to maintain my low supply. That first couple of thousandths inches in front of the base of the belt is an Achilles heel with magnums that can run the pressure up quickly like the weatherby--it's amazing how often I find cases that are fired once and are factory ammo shot in factory rifles that have expanded past specs, even while the primer still seats firmly.

[Bart B.]

For cases headspacing on their shoulder, setting their shoulder back a couple thousandths centers the neck on the shoulder so bullets are centered on the bore axis when fired. If the bolt face is not squared up, it also prevents the bolt from binding up when closed on out-of-square case heads..

What specs are those cases expanding more than; case or chamber?

Why is that first couple of thousandths inches in front of the base of the belt is an Achilles heel?

[stagpanther]

For cases headspacing on their shoulder, setting their shoulder back a couple thousandths centers the neck on the shoulder so bullets are centered on the bore axis when fired.

Ahh--so this is more about concentricity being "knocked out of kilter" by the force of the firing pin strike?

What specs are those cases expanding more than; case or chamber? Why is that first couple of thousandths inches in front of the base of the belt is an Achilles heel?

Case--with the result the case cannot chamber correctly in the rifle without resizing all the way down to the start of the belt. I understand that could be the result of one rifle's loose chamber specs vs another's tighter one.

[scatterbrain]

One of my first enlightments came was when I ran my fired cases through the concentric gage, I have never found a fired round to be out. Then where does it come from, just remove the expander from your sizing die and remeasure. A lot to be gained by knowing what your dies are doing.

[Bart B.]

Concentricity isn't knocked anywhere by firing pin impact.

All belted cases have that issue next to the belt. Chamber headspace is typically .220 +.008" and case headspace is .220 -.008". The case head and belt stretches back several thousandths to the bolt face when fired.

A solution: https://www.larrywillis.com/

[Bart B.]

People have been removing the expander for decades. They hone the die neck out to a couple thousandths less than the loaded round neck diameter. Thar causes about. 001" interference fit to the bullet.

Forster hones their own dies to customers specs for twelve bucks or so.

[stagpanther]

A solution: https://www.larrywillis.com/

That looks awesome! Thanks for the reference.

[stagpanther]

From his site, Bart:

My cartridges had swollen to .514" after the second loading of my cases. This dimension starts out at .507" on unfired Federal or Winchester cases. The "standard" case dimension in this area is .511" according to all of my reloading manuals.

This precisely nails the problem and is exactly what I see even in brand new ammo fired once, only I've seen that bulge even go out to close to .515" in some cases.

[Bart B.]

All chambers are not at SAAMI specs. The 300 Win Mag chamber can be .516" diameter at that point and within specs.

What cartridge are you referring to?

[stagpanther]

All chambers are not at SAAMI specs.

What cartridge are you referring to?

I see these deviations mostly in weatherby magnums, just like Larry observed, there is a "bulging" of the diameter immediately proceeding the belt and very typically it's in the .514" that he encounters. That is exactly what I've seen.

[Bart B.]

This is normal with all belted cases.

[Unclenick]

MG,

I am laughing because the drawing I altered had what you are asking for. It is here, and again, grossly exaggerated to the point the case mouth is in the lands: