Bullet run out how much matters?

[michaeljp65]

Admittedly unscientific sample size. 10 shots each, .308, 100 yards with a stock FN SPR

I continue to “feel” that run out is important, and the less there is the better. Of course, it could just be more reloading witchcraft.

Thats a good experiment Nature Boy showing a difference. Now, im not doubting anything you've done here, intact i commend you for taking the time to do it. But, what if you did the same experiment and shoot the .003 group first? Im thinking that the .001" group im assuming you shot first. Cold barrel, clean barrel. You may well have let the barrel cool right down for the other two groups as well. But it would be interesting to see if you get the same results if shot in a different order.
Cheers

 

[SWRichmond]

Mass produced sizing dies dont know how thick the case neck of "your" case will be, so they have internal dimensions guaranteed to smash down the neck of the thinnest case that might exist and still produce several thousandths of neck tension.

So if you're using thicker-necked brass, the case mouth is many thousandths of neck tension, and in my experience lots of neck tension causes lots of run out.

 

[Alex Wheeler]

And none use the case shoulder mid point as the front rest like it is in the chamber when fired. I'm amazed that nobody makes runout thing-of-a-bobs that does that. Maybe they're ignorant of what really happens.

Regarding bullet ogive uniformity, people who have used 50X optical comparitors comparing shapes across a production lot see excellent uniformity. Except one lot of Lapua 30 caliber 185 grain FMJRB Match bullets that had 4 different ogive shapes. Proof they came from 4 different pointing dies.

What really happens is the cartridge is supported by it's two tightest diameters, the bullet in its freebore diameter at the front and the case base at the rear. I think this may be why I believe runout has little effect. Theres only a couple ten thousandths that a bullet can "runout" from the freebore, because that is all the clearance it has. If I was loading short oal for magazines and the bullet was not supported in it's freebore then I would expect to possibly see effect of runout because at that point, the case shoulder would be doing some "aligning".

 

[Scott Harris]

What really happens is the cartridge is supported by it's two tightest diameters, the bullet in its freebore diameter at the front and the case base at the rear. I think this may be why I believe runout has little effect. Theres only a couple ten thousandths that a bullet can "runout" from the freebore, because that is all the clearance it has. If I was loading short oal for magazines and the bullet was not supported in it's freebore then I would expect to possibly see effect of runout because at that point, the case shoulder would be doing some "aligning".

And this is also why having "tight-neck" chambers doesn't improve alignment, but sure can cause other problems.

 

[Bart B.]

What really happens is the cartridge is supported by it's two tightest diameters, the bullet in its freebore diameter at the front and the case base at the rear. I think this may be why I believe runout has little effect. Theres only a couple ten thousandths that a bullet can "runout" from the freebore, because that is all the clearance it has. If I was loading short oal for magazines and the bullet was not supported in it's freebore then I would expect to possibly see effect of runout because at that point, the case shoulder would be doing the back of the case body some "aligning".

What this tells me is bullets are a couple tenths smaller than throats and the back of the case body the same amount smaller than the chamber.

What prevents the firing pin from driving the case shoulder hard into the chamber shoulder for consistent primer firing?

First time I've heard of this scenario.

Are clearances this small in all rifle barrels?

 

[Uncle Ed]

"a full-length sized case in which the neck is also fully sized. There is clearance at the neck and in the body of the case, the closest fit anywhere is the bullet in the throat. If the neck to bullet concentricity is good (although it needn't be perfect), then the bullet will find good alignment in the throat and the case body and neck will have minimal influence. Let's not forget that the base of the case is supported by the bolt face or the extractor to a certain degree as well; this is yet another influence on alignment. As you can see, there are several points from base to bullet that can have an effect. My procedure is to minimize the influence of those that I can control, namely the case body and neck, and let the alignment be dictated by the fit of the bullet in the throat and to some extent by the bolt's support of the base. Barring a seriously out of square case head, I don't think the bolt can have a negative effect on alignment, only a slightly positive effect from minimizing "case droop" in the chamber. Given that a resized case will usually have a maximum of 0.001" diametrical clearance at the web, this isn't much of a factor anyway.

In conclusion, I believe that allowing the bullet to find a relatively stress-free alignment in the throat by full length sizing (including the neck) and turning necks to enhance concentricity gives the bullet the best probability of a well-aligned start into the rifling."

The Rifleman's Journal
Germán A. Salazar

 

[Alex Wheeler]

What this tells me is bullets are a couple tenths smaller than throats and the back of the case body the same amount smaller than the chamber.

What prevents the firing pin from driving the case shoulder hard into the chamber shoulder for consistent primer firing?

First time I've heard of this scenario.

Are clearances this small in all rifle barrels?

The firing pin can and will drive the case forward but the bullets in the freebore in most cases and its not changing that. Many saami spec reamers dimensions do not go much over .001" nominal bullet diameter. I would expect very precise bullet alignment to improve accuracy. However, in practice, even loosening the freebore diameter specs has not harmed accuracy, in fact I believe its improved it. I also believe that the neck opens to release the bullet mostly. I do not believe the bullet slips out of the neck in the same fashion its seated. If that was the case, we would see necks that are not fully expanded as pressure could equalize on the out side of the neck. That said, if thats true, which I believe it is, then all of the concentrity goes right out the window at the instant pressure builds and the necks starts to release the bullet. There are many scenarios, but in this case I am speaking in terms of a rifle built for accuracy. In the case of a sloppy factory chamber, I have no interest or opinions.

 

[Scott Harris]

The firing pin can and will drive the case forward but the bullets in the freebore in most cases and its not changing that. Many saami spec reamers dimensions do not go much over .001" nominal bullet diameter. I would expect very precise bullet alignment to improve accuracy. However, in practice, even loosening the freebore diameter specs has not harmed accuracy, in fact I believe its improved it. I also believe that the neck opens to release the bullet mostly. I do not believe the bullet slips out of the neck in the same fashion its seated. If that was the case, we would see necks that are not fully expanded as pressure could equalize on the out side of the neck. That said, if thats true, which I believe it is, then all of the concentrity goes right out the window at the instant pressure builds and the necks starts to release the bullet. There are many scenarios, but in this case I am speaking in terms of a rifle built for accuracy. In the case of a sloppy factory chamber, I have no interest or opinions.

why do you think a larger freebore vs bullet diameter increases accuracy?

 

[Bart B.]

German Salazar sez:

Barring a seriously out of square case head, I don't think the bolt can have a negativeeffect on alignment, only a slightly positive effect from minimizing "case droop" in the chamber.

He forgot to mention the extractor can push the case body rear end against the chamber wall. And inline ejector's can push cases full forward in chambers like firing pins do.

Case droop?

 

[Bart B.]

I do not believe the bullet slips out of the neck in the same fashion its seated.

Why not? Especially if it's been seated for only an hour or two. Ever measured?

If a 30 caliber bullet needs about 2 pounds of force to seat it, it will start moving out of the case neck when pressure in the case is about 27 psi. A 22 caliber bullet the same, it'll start moving out at about 51 psi. They slow down as they get engraved, then speed right up.

I don't think case necks will expand much forward of their heels. And very little behind the heel.

 

[Alex Wheeler]

One reason is that seating force is mostly a friction measurement, it shows up big on the dial but actual tension is what shows on the target. The other is when you have a rifle with a very long freebore and have very little bullet in the neck, you may see the neck not expand at all when fired, in that case the bullet did slip from the neck allowing pressure to equalize around it. I also can't agree with the idea the bullet slows as it hits the lands. Theres so much pressure behind it that I dont believe it slows down at all. Its rate of acceleration may plateau in some un measurable amount, but no way it slows down.

 

[Alex Wheeler]

why do you think a larger freebore vs bullet diameter increases accuracy?

Wish I knew. But we know accuracy suffers when we get too tight around the neck as well. I had a .30 cal with .002" neck clearance that would not shoot and I could not get es under control. Increasing it to .004" cured the problem. .002" should be enough and in some cases it is. I dont think its a case of more clearance being more accurate, rather not enough being less accurate.

 

[Uncle Ed]

German Salazar sez: He forgot to mention the extractor can push the case body rear end against the chamber wall. And inline ejector's can push cases full forward in chambers like firing pins do.
Case droop?

The rear of the case is held and centered by the recessed bolt face. Case droop is gravity pushing down on the cartridge and Mr. Salazar explains "Given that a resized case will usually have a maximum of 0.001" diametrical clearance at the web, this isn't much of a factor anyway." Sometimes explained by "the cartridge should fit the chamber like a rat turd in a violin case". Meaning the full length resized case body has no effect on bullet alignment with the axis of the bore.

 

[Scott Harris]

Wish I knew. But we know accuracy suffers when we get too tight around the neck. I had a .30 cal with .002" neck clearance that would not shoot and I could not get es under control. Increasing it to .004" cured the problem. .002" should be enough and in some cases it is. I dont think its a case of more clearance being more accurate, rather not enough being less accurate.

Ah, neck not FB. I totally agree, like .006 clearance in the neck.

 

[Bart B.]

I also can't agree with the idea the bullet slows as it hits the lands. Theres so much pressure behind it that I dont believe it slows down at all.

I don't think there's much pressure in my my 2 examples with each bullet .001" short of touching the lands, 27 and 51 psi.

 

[dmoran]

If a 30 caliber bullet needs about 2 pounds of force to seat it, it will start moving out of the case neck when pressure in the case is about 27 psi. A 22 caliber bullet the same, it'll start moving out at about 51 psi. They slow down as they get engraved, then speed right up.

What are you basing your release pressure amounts from?
If it's a mathematical formula, what is it?

As to bullets slowing down when engaging the barrel: from actual pressure traces, I have seen that to occur occasional, dependent on circumstances, but more typically will not see it occur. Bullet entrance pressures, are dominantly controlled by neck tension and/or neck preparation steps, that can be easily tuned from about as low as 2500-psi to +10,000-psi by either one and/or combinations of both. Between 5000 to 8000-psi being a typical pressure range for bullet entrance.

 

[Alex Wheeler]

Ah, neck not FB. I totally agree, like .006 clearance in the neck.

I was only using the neck as another example of more clearance giving better results similar to what I see in the freebore.

 

[Bart B.]

The rear of the case is held and centered by the recessed bolt face.

Are all case head's recessed bolt faces the exact diameter of case rims? Surely, there are tolerances. And in bolt body and receiver boltway

SAAMI specs have about .010" tolerance on case rim diameters.

Don't think this is reality. Never come across such stuff before. Unless it's a secret and you just leaked it out.

 

[Bart B.]

What are you basing your release pressure amounts from?
If it's a mathematical formula, what is it?

Let me check with the gun God to see if he thinks you're worthy of knowing what the simple grade school math formula is.

 

[Bart B.]

Case droop is gravity pushing down on the cartridge and Mr. Salazar explains "Given that a resized case will usually have a maximum of 0.001" diametrical clearance at the web, this isn't much of a factor anyway."

So the case body doesn't touch the chamber anywhere because of diametrical (radial) clearance all around the case?

Or does he mean the case body rests on the bottom of the chamber?