let's break this down.

[Paden]

I think you got it except the powder has to be igniting before the bullet starts moving or you get that stuck bullet pressure rise SEE event.

one more thing that will help get the powder burning better is the bullet engraving the rifling.
this will give a momentary pressure spike you can see on trace equipment so the powder has to be going by then.
I have not seen the primer pop on trace equipment [or I haven't been looking close enough to see it]
but there is always a pressure rise before the little bump [and drop down back to the pressure line] the engraving gives.

Let's use a revolver as an example (or for that matter, a rifle with factory or other load which for whatever reason has jump to the lands, as is the case for most loads aside from the specialty cast/sized for throat niche)... My 45 Colt loads use the 347 grain bullet pictured to the left; they're seated with lots of neck tension and a very strong crimp in the crimp groove. And they are sized to firmly fit my cylinder throats. As verified yesterday, the primer pressure alone is easily adequate to move the bullet against that resistance into the forcing cone. It's evident that it's really not possible to hold a bullet in a case with just neck tension/crimp. Thus the question is... Does the powder ignite before, during, or after the bullet starts moving from the primer pressure; before or after it stops moving from hitting the resistance of the lands; on a micro scale, what does that first millisecond of the pressure curve look like?

In a perfect world, my hope would be that the powder ignites at the same time, if not before the bullet starts moving; once it starts moving it continues to smoothly accelerate, and does not hesitate at the lands... But...(?).

 

[freebullet]

Playing with surplus imr7383 in every rifle caliber I own has taught me a bit about the timing events. Not that I'm any kind of expert on it.

In x39 for example a primer will push the uncrimped boolit out fast/far enough that you'll find a case full of burned but, not moved or completely burned powder. It looks just like you loaded it but is clearly burned and crumbles when you touch it. A reasonable crimp clears that right up.

Lest we not forget the need for a good crimp on heavy revolver loads with slow powders. Not only to prevent jump/lock up but, it helps make sure the powder is lit before the boolit moves.

A win spp will break the crimp and lodge a 140gr boolit in the barrel of a 38spcl. It did not clear the 2" bbl. With fast powder it matters not but, with slow for caliber it could lead to a case full of partially burned or unignited powder.

In 357 without a good crimp slow powders have burned dirty/incomplete for me. Finding that perfect balance of crimp can make similar changes as warmer/longer burning primers.

In the 336 when I seat so the lever forces the boolit in the throat a win lrp will not move the boolit forward with any crimp at all & no powder. Magnum primers from s&b would break a light crimp.

Sometimes loading a squib on purpose can show quite a bit before you load any powder. That practice would make many cringe however, sometimes thinking & testing outside the box gets us answers we seek. A boolit moving forward before trying to light slow for caliber powder is not going to get consistency ime.

 

[Brad]

Good question. I would think in a revolver it is a pretty smooth curve. Primer starts the bullet moving as it also ignites the powder. As the bullet hits the forcing cone pressure should be rising and powder burning well. I wouldn't think a spike in pressure occurs as the pressure is adequate to engrave the rifling and pressures peaks.

I do think that a weak crimp/tension can cause hang fires or squibs in revolvers. I had some in my 44 mag with WC820 and a 429421. Bullets lodged in barrel with a mass of unburned powder behind it. I increased the crimp and squibs stopped. I think the weak crimp let the bullet move enough to drop pressure below the point where good ignition could be maintained. This was because of weak neck tension and the crimp was trying to make up for it.

What is scary in a revolver is when the same thing happens except the powder regains enough pressure to burn well and the reigniting leads to a pressure spike.

 

[Ian]

So, what we've really learned here in rifles at least is A difference between jacketed and cast accuracy might lie in the difference in engraving load that the powder has to work against, and how consistent that engraving load is between dry copper and lubed lead alloy.

I know the alloy has to be just right to bump the powder burn and also bump the bullet if you want to get accuracy at HV, and bullet shape plays into that a bit, too. I did a bunch of tests with the original .30 XCB and the 30-190X cast of various things, along with two other bullet designs. Matching the throat perfectly and seating the bullet right up against it doesn't work as well as one would think, regardless of alloy used. By the time case pressure is enough to move the bullet against all that engraving resistance, the neck has blown out to the chamber wall leaving the bullet's bearing bands hanging in space with flames all around. Not good.

Throat/bullet mismatch in the right places to cause engraving pressure vs. forward movement distance to be graphed as a curve instead of a brick-wall vertical line is just about essential to accuracy with fixed ammunition (at my house, anyway).

 

[Brad]

A bullet in motion engraves the rifling easier than one sitting firmly against the rifling. Easier to throw a rock thru a window that to push a rock thru a window.

Having the bullet move smoothly into the lands is good. If it stops, even for a tiny bit, the back end is still in motion and gets riveted a little. A bullet design that lets the engraving occur over time rather than abruptly also would help keep the forward motion smooth.

 

[fiver]

a big smile just went across my face, now THIS is a reloading discussion.
everybody is thinking, and starting to put things together.

 

[Brad]

I asked my daughter to model it but she doesn't know how to do dynamic finite element analysis.
She did agree that a bullet like the 190x hard on the lands will have more bullet deformation, riveting, than one allowed to hit the rifling with a running start.

I gotta get her modeling this stuff for us. I need my money's worth out of that education.

 

[Paden]

A bullet in motion engraves the rifling easier than one sitting firmly against the rifling. Easier to throw a rock thru a window that to push a rock thru a window.

Having the bullet move smoothly into the lands is good. If it stops, even for a tiny bit, the back end is still in motion and gets riveted a little. A bullet design that lets the engraving occur over time rather than abruptly also would help keep the forward motion smooth.

Agreed. And logically, the harder the alloy/the more drag, the more important the running start (at least within the context of maintaining a nice smooth pressure curve and acceleration).

I think I know precisely what I want to happen after the 'click'; I just wish I could see inside that brass to confirm/understand EXACTLY what's going on.

 

[freebullet]

Lol, before ya know it she'll be developing custom cast only wildcats.4k fps will seem slow for cast when she's done.

What your saying there makes sense though. Like seating Jax on the throat. Definitely increased pressure.
Finding that launch balance is the trick.

Maybe she could invent high speed nanotechnology cameras that we could fire. Now that would be something. ...

 

[Ian]

So we're trying to do two things at once really in that first half-inch: Get the right pressure start and build for blast-off, and get the bullet straight into the bore.

But there's so much TO that and the two operations are inter-dependent. Dynamic bullet fit (alloy, shapes, displacement areas) affects the burn just like the burn and resulting pressure rise is the motor behind the "dynamic" in the dynamic fit part. While all that is happening, several different barrel harmonics will be established which will matter a lot at the other end of the tube. And in the meantime we have to make sure we did everything exactly the same at the loading bench each press pull so all the click-boom happens just the same every time.

Something that always struck me as odd was how the jacketed BR guys measure powder. Most of the time it's like a Justin Wilson recipe: Meh, just toss "about that much" into the case and done. Later in the morning when the temperature is up a few degress, toss in a little less. Why? Because exactness to the 1000th of a grain doesn't matter, even at 200 yards where group measurements have a zero after the decimal point. Get things dialed in and there's a relatively huge range within which the system will respond the same. Now think about that.

 

[fiver]

it's like shooting a ladder where the different powder charges overlap each other.
once you hit that zone your need to weigh every charge is over, .2 here or .2 there isn't gonna affect your group size all that much.
it's time to tune the oal the make up the difference in the ignition sequence.

 

[Tony]

In one sense the jacketed BR guys do what everyone else does. They use a highly precision powder measure to throw a volume of powder. They DO NOT obsess over what that volume of powder weighs. A lot of them do not know or care what the volume of, say, VV N 133 that they're using weighs. They just know that "26.2" clicks in "their" powder measure will be the same. Their measures are highly precision instruments that are repeatable.

As a group, 6 PPC shooters tend to vary powder volume slightly to suit changing conditions. 30 BR shooters do not as that cartridge seems to be unaffected by conditions encountered at a given match. I have known some 30 BR shooters, assuming they had enough prepped brass, that would pre-load and show up at the match with all of their ammo already loaded.

 

[fiver]

So we're trying to do two things at once really in that first half-inch: Get the right pressure start and build for blast-off, and get the bullet straight into the bore.

But there's so much TO that and the two operations are inter-dependent. Dynamic bullet fit (alloy, shapes, displacement areas) affects the burn just like the burn and resulting pressure rise is the motor behind the "dynamic" in the dynamic fit part. While all that is happening, several different barrel harmonics will be established which will matter a lot at the other end of the tube. And in the meantime we have to make sure we did everything exactly the same at the loading bench each press pull so all the click-boom happens just the same every time.

Something that always struck me as odd was how the jacketed BR guys measure powder. Most of the time it's like a Justin Wilson recipe: Meh, just toss "about that much" into the case and done. Later in the morning when the temperature is up a few degress, toss in a little less. Why? Because exactness to the 1000th of a grain doesn't matter, even at 200 yards where group measurements have a zero after the decimal point. Get things dialed in and there's a relatively huge range within which the system will respond the same. Now think about that.

remember discussing using alloy, and boolit jump or lack of it to manipulate a too slow powder.
now I think we understand how that spike is working for/against us, and why using a longer jump to the lands allows us the use of a slightly faster powder.

 

[fiver]

alright lets move on to burn rate I know we touched on it and when/how it starts burning when the primer ignites.
this one is going to require a LOT more thinking even though it seems simple.
one example I like to use is the difference between 700-x and 800-x.
there isn't a difference in their make-up they are the same exact powder.
okay.... what?? they have different burn rates.
yeah they do, kinda.
the difference is in their flake size.
700-x is smaller easier to ignite and you burn through it faster.
the bigger diameter and slightly thicker 800-x takes longer to burn so it produces gas longer.

 

[Paden]

the difference is in their flake size.
700-x is smaller easier to ignite and you burn through it faster.
the bigger diameter and slightly thicker 800-x takes longer to burn so it produces gas longer.

...Tho, the larger has more surface area. So.....coatings/inhibitors vs. shape/size/surface area vs. density (of the powder itself) vs. density (of the load)...

...And now some have figured out how to mix inhibitors throughout the powder, so they're less dependent on surface coating and shape to control burn rate. Which is VERY cool.

 

[KHornet]

This thread has become way to tech for an old codger like me!

 

[fiver]

Hornet ask questions we like questions.

one more thing they do is make holes in the powder some just have a hole through the center, others like rl-17 [which is 'progressive burning'] have holes throughout them, this exposes more and more surface area as it burns.
some powders are coated with burn deterrents, which is another way of using the same formula to make 2-3 burn rate powders.
and others manipulate the diameter and length of the sticks to control how fast they burn.

 

[KHornet]

Nope, will just observe and maybe learn.

 

[JWFilips]

Guys:
Same here! I'm taking it all in but have nothing of scientific nature to add! However I'm following most of what you are saying ( makes me rethink my "on the lands" load start procedure)
So I'm taking it in and learning
Jim

 

[yodogsandman]

So, how should we choose a powder? There's no HV cast loads listed in the manuals. The burn rate charts are just kind of a general guide, for the most part. Without a vast knowledge of how a powder burns in a specific case, we're left floundering. Sometimes, I've just been going by the slowest powders listed for jacketed loads but, that really doesn't work either. I've been trying slower powders than even those listed for jacketed. Because of the slipperiness of a cast bullet, using data from jacketed bullets of less weight to approximate the drag of friction in the bore.