Bullet Length in Relation to OAL

[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:

 

[scatterbrain]

Unclenick, what you have pictured would take a serious effort to accomplish and be very messy after firing. How much pressure would be required to release that bullet?

 

[Metal god]

Unclenick, what you have pictured would take a serious effort to accomplish and be very messy after firing. How much pressure would be required to release that bullet?

That would not happen , see below my opinion on that . I'll put that part in bold print .

I am laughing because the drawing I altered had what you are asking for.

Are you saying the first exaggerated image you posted shows what I was trying to say ?

If so I disagree , even though the head clearance is significant in the lower image . Both bullets engage the lands at the exact same spot when pushed fully forward . The image above says to me no mater how short you size your case head to datum the bullet will engage the rifling at the same time and distance . Maybe I'm wrong but I thought the whole point was the opposite of that . Meaning if one shoulder is bumped back .005 shorter then the next and both cartridges have the same COAL . The cartridge with the shorter shoulder bump will have it's bullet ogive .005 closer to the lands then the other after firing pin or ejector pushes them forward ? If the above image showed that the bottom image would have the bullet jammed into the lands ???

Your second exaggerated image does however

Although I don't think you need the exaggerated longer case neck being jammed into the rifling . Reason being IMHO that is not likely to happen because in most instances if you are at "trim to length" that extra .005 bump or even as much as .010 shoulder bump and the cases trimmed to length . The neck will not be pushed into the throat/leade/rifling . It's the very reason we have all the tolerances we do between case size and headspace size , to allow for the reloader to have inconsistent head to datum distances . However the ogive of the bullet will get closer to the lands which is what we are talking about here . At least that's what I've been talking about

 

[stagpanther]

Camming onto the bullet with the case mouth into the throat is a good way to bulge the brass or even potentially blow up a rifle--though I'm not sure that's what UN was intentionally showing in the revised exaggerated pics?

 

[Metal god]

-though I'm not sure that's what UN was intentionally showing in the revised exaggerated pics?

Agreed but it does show what could happen if you fail to trim "and" bump your shoulders back to far . It could happen , obviously not as much as the image shows but the tolerances can stack up and you could jam the neck into the throat . So although not likely IMO , I can see it happening .

In fact this makes me think that may have been happening to me when I first started reloading . I was getting sticky bolt lefts at start charges or just above . I now load those same components to much higher charges with out issue . There may be a couple things different now compared to when I started . However the glaring one is that I adjusted my dies per instructions which had me bumping my shoulders back .012 from fire formed . At the time I did not know that , I was just sizing my cases as instructed by the die manufacture . Obviously those rounds chambered just fine but maybe I was jamming the case neck into the throats . Interesting thought for me to contemplate , I guess some more testing and measuring is in order . Maybe I'll get to that just after I finish that primer kill test I never started

 

[stagpanther]

Agreed but it does show what could happen if you fail to trim "and" bump your shoulders back to far . It could happen , obviously not as much as the image shows but the tolerances can stack up and you could jam the neck into the throat . So although not likely IMO , I can see it happening .

In fact this makes me think that may have been happening to me when I first started reloading . I was getting sticky bolt lefts at start charges or just above .

Actually, I believe I did this very thing today with one cartridge of the very first groups I shot through my howa 300 wm after fire-lapping the bore with tubbs finishing bullets. I posted a pic of the result on the bolt action sub forum.