"Percent Fill" effect on accuracy

H

Harold M

New member
Quoted from: VARMINT HUNTER MAGAZINE – WINTER, 2012 PAGE 168
“Any charge near a 100-percent filling ratio is likely to be inaccurate. Decades ago, the late Mr. Delaney, working as chief ballistician at Hercules, recognized this problem. Any such load is apt to create cartridges of two basic types: first, those where the charge settles a bit more than average and where the charge is therefore uncompressed after bullet seating; second, those where the charge settles a bit less than average and where the charge is therefore compressed during bullet seating. Cartridges from such loads are well-known for shooting into separate groups.”

I had never heard this before. Is this "old news?"

Harold
 
I have no idea for a factual basis but sometimes it does happen to me but not consistently.
 
Seems to me that it would be as easy to identify each condition retrospectively as it is to be adamant on the family affiliations of fossil species.

As it happens, I've been loading the .308 case at a around 100% & plus density for a couple of decades now for 300-1200 yard belly matches without recognizing atypical grouping. That included loading & carting ammo a couple thousand kilometers across Australia by car & over to Britain using air transport.

I would be far more concerned about projectiles backing out randomly when excess compression is attempted. I've certainly seen that occur, though not in my personal loads.
 
My thoughts -----

It is better to have the bullet resting on top of the powder "IF" the powder is slowly droped with a long drop tube. However its BEST to tune to the conditions of the range at that time. That being said, my normal load just starts up the neck by loading in that manner. I have got to a match and found that a slightly compressed charge shoots better. I adjust to whatever I can get to shoot at that paticular time if it be a little more or a little less.
 
Maybe???

Experience on my part has not shown this phenomenon. Logic wise it makes a lot of sense. Now there is the matter of whether or not words were put in this ballistitions mouth. I think considering that all this guy does is burn powder in every possible senario all day every day in a hi-tech lab has me wondering if it isn't true to some degree...
 
Well yeahh but...... as you said, he "burns powder all day." There are very few people or situations capable of testing this sort of thing. Unless the difference shows up in INCHES normal rifles will never show it consistently and I don't believe it's inches of difference.

al
 
Just a guess but, this is probably more evident with long straight cases with larger bullets relative to case diameter where flame propagation starting at rear of case ignites more easily than powder that is running up behind the bullet (and possibly compressed) resulting in a widening of the pressure curve and moving peak pressure further out in the time line. I think short stocky cases with small bullets are more efficient.

Ken
 
K Hope said:
Just a guess but, this is probably more evident with long straight cases with larger bullets relative to case diameter where flame propagation starting at rear of case ignites more easily than powder that is running up behind the bullet (and possibly compressed) resulting in a widening of the pressure curve and moving peak pressure further out in the time line. I think short stocky cases with small bullets are more efficient.

Ken
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Well yeahh but....... that's not how powder burns :)

At least not from what I've read. It ignites and immediately swirls and whirls inside the case, it doesn't burn from back to front, each grain is generally involved and burning before the first grains are consumed.

al
 
If a load is 100% density, the powder completely fills the space available with enough left over to seat a bullet on it, I doubt that there's much settling with the fairly fine grained powder used in BR. I've never seen what the old Hercules rifle powders looked like, but if they had big long granules like the old IMR powders do and the powder wasn't carefully settled it might make some difference. But if those loads were prepared in a consistent manner, or transported any distance in a vehicle of some kind that should settle the charges pretty uniformly.

Not something I'm going to spend any time worrying about that's for sure.
 
alinwa said:
Well yeahh but....... that's not how powder burns :)

At least not from what I've read. It ignites and immediately swirls and whirls inside the case, it doesn't burn from back to front, each grain is generally involved and burning before the first grains are consumed.

al
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Well hey Al we do hot have to agree on everything. :)

Flame propagation is a function of pressure; a high load density results in an earlier and a higher peak pressure than a lower load density. However, the volume into which the gasses are moving is increasing and this decreases pressure. The bigger case / bore combination, the faster pressure drops since the volume is greater. This translates into the all burnt moving more towards the muzzle. As I see it a short stocky case with the primer in the rear is ignited much more thoroughly and proportionately than a long case big bore.

Sorry I do not know anything about the swirls and whirls effect. :) But in general, I do agree on the bulk/mass burn wave process.


Ken
 
I have read best loads for pressure and velocity consistency and therefore accuracy are achieved with a proper powder charge that fills the case without being compressed. A nearly full case keeps the powder in a uniform column above the primer resulting in very uniform ignition. A 2/3 full case allows the powder to spread out along the bottom side of case leaving an air gap at the top side when the rifle is horizontal.
 
What is it that comes into play when someone uses a load that is too small and as has been reported on this site, causes extreme pressure issues?
 
Vern, from what I've read and sounds reasonable is that light charges of slow burning powder the powder granules shatter and since the deterrent coating is on the outside of the granules the powder burns MUCH faster.

By mistake (can't read apparently) I loaded some H4831 in my 6 BR with H4895 charge weights and light bullets. Nothing evil happened except for some wasted bullets, powder, and primers not to mention VERY low velocities, but the cases were nearly full of powder.
 
Not too sure that I've ever shot a 6 PPC load that wasn't a full case of powder once the bullet was seated. Some fuller than others, but just about all with some degree of compression from the bullet.
 
well you need to do some real world number crunching to go with your generalities.....
a 6ppc with a full case of n133 hits peak pressure just past one inch of bullet movement.....its not flame propagration...but actual burn.....as al said it does not just start in the back and move forward in a straight line as the word propagation implies....it just aint so

mike in co
..........
K Hope said:
Well hey Al we do hot have to agree on everything. :)

Flame propagation is a function of pressure; a high load density results in an earlier and a higher peak pressure than a lower load density. However, the volume into which the gasses are moving is increasing and this decreases pressure. The bigger case / bore combination, the faster pressure drops since the volume is greater. This translates into the all burnt moving more towards the muzzle. As I see it a short stocky case with the primer in the rear is ignited much more thoroughly and proportionately than a long case big bore.

Sorry I do not know anything about the swirls and whirls effect. :) But in general, I do agree on the bulk/mass burn wave process.


Ken
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Mike Bryant said:
Not too sure that I've ever shot a 6 PPC load that wasn't a full case of powder once the bullet was seated. Some fuller than others, but just about all with some degree of compression from the bullet.
Click to expand...
Mike, I dunno if you ever used the Israeli 2015-BR back in the mid-90s, but it was my favorite powder. Cases would be full, but there would still be just a bit of airspace in them. Use to use Norma cases with that powder too -- it didn't much need high pressures.

Shooting a lot of .600 to .800 B.C. bullets at or over 3,000 fps cured me of any notion that I could "shoot through" the wind. YMMV.
 
mike in co said:
well you need to do some real world number crunching to go with your generalities.....
a 6ppc with a full case of n133 hits peak pressure just past one inch of bullet movement.....its not flame propagration...but actual burn.....as al said it does not just start in the back and move forward in a straight line as the word propagation implies....it just aint so

mike in co
..........
Click to expand...


Burn Rate function sequence:

The first event, ignition, depends on the propagation of a flame front, generated by a priming source (at the rear of case) and burning of those propellant grains ignited first, through the propellant bed.

The second event, a regressive burning of the propellant grain.

I do not consider these as generalities but rather inherent properties.


Disclaimer: My knowledge of thermodynamics is lower than low and my shooting skills are about the same.


Have a Good Day and That’s all……..

Ken
 
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