let's break this down.

[fiver]

hmm weight seems to be a factor, I used my Dillon 550 set up with the powder measure emptied but normal load procedures otherwise.
I seated the boolit deeper in the case on my summit press to see what difference it made and it went just as deep but seemed harder to knock out of the barrel.

 

[fiver]

so what does that tell us about primers knocking everything [powder] around in a case if it can move 200-350 grains of weight around?
5 grains of red-dot doesn't stand a chance, and it better be making gas almost right now.

tells us a little something about Ian's subsonic 308 load.

 

[Ian]

This is some interesting stuff. Gotta go eat dinner and I'll share a couple more squib experiences pretty much the same as those you guys are doing.

 

[fiver]

Brads goofing off with people from work and is missing all the fun.

I expected the boolits to be shoved out of the case.
but i was surprised at how dirty the inside of the cases were, it was that same black crud win primers leave behind in the primer pocket.
only it coated the inside of the case and the back of the bullet too.

 

[Paden]

Brads goofing off with people from work and is missing all the fun.

I expected the boolits to be shoved out of the case.
but i was surprised at how dirty the inside of the cases were, it was that same black crud win primers leave behind in the primer pocket.
only it coated the inside of the case and the back of the bullet too.

I have in the past had opportunity to pop off batches of primers in empty brass, and can attest that they will make your gun FILTHY. I'm quite sure that most all the crud left in my cases after shooting is a product of the primers, not the N110 powder.

 

[Ian]

I was going to mention that....and I think most of the gritty crud we see is primer residue. The only (knock on wood) unintentional squib I've had happened just recently in my 45 ACP AR-15 while I was doing something not smart which is working up a load on a progressive press and fiddling with the powder measure as I went. Anyway, the gas port is in the throat just ahead of the chamber. The squib just went "click" and I pulled the charging handle to eject an empty, sooty case. Hmmmmm. So I take a look and sure enough, the bullet's lodged in the throat. Apparently the bullet base stopped right past the case port and was in far enough to have rifling fully engraved on it. No gas cutting of the base happened at the port, and the lube was 100% perfectly in the groove and intact. All that answered a lot of questions for me about what happens during launch.

Anyway, inspired by William Jennings and Ken Mollohan, I embarked on the .30-30 squib load quest back in about 2010 and found "how low you can go" with Red Dot and Bullseye and large pistol primers. Stuck a few and tried to see how far up the bore different primers would push the bullet. All of them made it out of the case, but not much farther. I also learned to push the bullet through the way it was headed when it stopped, don't try and beat it back through toward the chamber. Primer grit behind the bullet is the main reason I think they don't like to go in reverse. Got a lot of backed-out or severely flattened primers due to a little excess headspace, which I cured with a thin o-ring.

Starmetal Joe and I were talking one time about our Ruger Old Army cap'n'ball revolvers and the stupid things we did with them. I used to shoot two grains of Bullseye in mine with a pretty dense Dacron wad between it and the bullet. (DON'T do this, anyone, there's a VERY good reason not to!). Well, he tries it and can't get any of them to go off. I never figured that one out, because CCI #11 caps made Bullseye go bang in mine on several different occasions, but the caps wouldn't light the powder in his. Maybe I was using different nipples or he was using weaker caps, not sure. Anyway, a #11 cap will blow a ball clear of the chamber and into the forcing cone on mine with no powder in the chamber.

One of the things I keep reading about SEE is low loading density and powders that are heavily deterred ("slow" rifle powders). It stands to reason that the primer flash might just blow the stuff around in the case like a leaf blower and maybe nudge the bullet forward some, and then some embers finally make it through the coating and a few powder kernels begin to ignite. Problem is all the powder is now very hot and near the combustion point when it all starts to go, and just like throwing a match after some kerosene thrown on a smoldering pile of coals, it goes KABOOM! all at once with little to no deterrent coating left to slow it down. To make things worse, the bullet has stopped moving somewhere in the throat and has become a bore obstruction. Basically the smokeless powder becomes dynamite in a pipe bomb and blows up the gun.

Faster-burning, less heavily-deterred, flake powders like a lot of the pistol and shotgun powders must blow around like so much confetti in a big rifle case when the primer goes off, but such powder lights a lot more easily and still burns progressively though quickly, and doesn't let the bullet stop in the throat (where it's hard to get started moving again) before the pressure peaks again behind it. I'm still not sure what exactly happens inside a case with something like 80% density medium to slow rifle powder when the primer goes off, but I suspect a whole lot of turbulence and heat is blasted through the whole powder column and the whole charge is exposed to pretty much the same primer heat and pressure throughout.

One thing I DO know is with slow powders, the closer to 100% loading density I get, the more consistent the load. The more consistent, the more accurate. Gotta look at both the chronograph statistics AND the group statistics to see the whole picture, though.

How my most accurate load can have a higher SD and ES than loads near it still baffles me.

 

[fiver]

so putting this together with neck tension is beginning to make a little more sense to me the more I think about it.
I still need to picture exactly when the case swells up, but I can see it happening more like Ian suggests where it all expands at one time.
it also explains sooty necks and stuff like lube/Dacron getting behind the neck or being caught on the case mouth.
and it also explains how bolt thrust works, since if the powder is not all contained in the case when it burns it will push backwards more than I thought.

I do know that stick powders travel down the barrel while they are still burning and they bounce off everything in their way.
I'm gonna have to do some more experimentation with some different case shapes now.
I wanna see how the double radius shoulder and the Ackley 40* shoulder handle this versus a normal 20ish* neck angle.

 

[Ian]

P.O. Ackley explained his theory of making the shoulder steeper and case walls more straight in one of his books. I wish I understood it better, but what I remember is basically it has to do with combustion efficiency and vectoring the burning powder back on itself for just a bit longer rather than trying to funnel so much of it out of the case right away while it's still trying to catch on fire. Looks like the PPC guys and Super-Duper-Extra-Short-Magnum developers were chasing the same rabbit. Wasn't Weatherby trying to reduce throat erosion with the double-radius chamber shape?


I used to wonder why a relatively short, straight-walled, steep-shouldered .308 case would do everything an '06 would do at the same pressure levels with the same bullets.....up to about 170 grains. Then the '06 starts to pull away and can shoot bullets over 220 grains with ease. I finally decided it has to be because the '06 strings out the pressure curve farther down the bore and makes the same powder burn longer than a .308. Case shape has to have as much to do with it as volume. You don't see many 16"-barreled '06s, do you? And if you did, I'll bet an equal-length .308 with the same powder and bullet will push the bullet faster.

 

[Brad]

http://6mmbr.websitetoolbox.com/post/throat-sandblasting-theory-3362471

Not just efficiency but also throat life. Shoulder angle and neck length could have an impact on throat life.

 

[fiver]

the way Parker explained it to me the shoulders on his cases rebutted the powders flame front, and actually sped up the burn rate of the powder.
he really picked it because it grew the case size and because it stopped case growth, the other is a side affect of the case shape.

the double radius was actually put there because Roy Weatherby was a jerk and he didn't want gunsmiths copying his case design.
seriously it had nothing to do with anything else.
it may affect the powder burn and deflect gas flow but it wasn't done on purpose.

shoulder angle also has an affect on throat life, airc you draw a line along the shoulder down into the neck and if it hits brass it won't allow as much powder to flow down the barrel and also deflects gas heat and such away from the barrel.

 

[Tony]

You guys have wandered off into a discussion of turbulence point.

 

[Rick]

Not only shoulder angle and neck length but powder also. Some powders burn hotter than others and a good example is 2520 which is fairly tough on throats.

 

[Ian]

Same idea with suppressor baffles where cross-jet designs contain the gas longer by making an obstruction to the volume behind each jet.

 

[Brad]

You guys have wandered off into a discussion of turbulence point.

We are world renknowned thread drifters.

This is a pretty typical discussion. At some point we may even discuss primers again.

 

[Missionary]

Greetings
Primer is what we used to dump down the pump to get the leather washers to suck up the water... that was my first introduction to a primer. But then I digress back to primers. Must be the altitude today or barometric pressures.
Mike in Peru

 

[yodogsandman]

The "powders turbulence" after bullet release depends on each different powders shape and cartridge neck angle, effecting the primers uniformity? So if using either a tighter neck tension or using a crimp, would it also slow down the bullets release a tick from the case, allowing the primer flash to ignite the powder more evenly or more fully? That's a lot of scenarios!

 

[fiver]

yeah we are good at wandering in our thoughts, but the more I think about it the more the primer/case size/and powder fill is affecting things.
I knew different primers definitely affected group size, but does it pop the bullet free in every case or is it just the shorter cylindrical cases.
if you popped the bullet free in a 5.56 round and followed it up with 60-k of pressure with nothing to protect right in front of the case you would get gas cutting or severe erosion right there like gas cutting on the frame of a revolver.
surely the longer powder columns have to protect the bullet from this happening.
and I seriously doubt it happens in a shot shell there is too much cushion and weight for the primer to move all of that.
although it could explain why efficiency was improved when the plastic wad come around versus the old cardboard and cork height adjusters.

so what I am seeing happen is the primer is actually introducing gas/pressure/flame inside the case, sometimes it is enough to move the projectile
[like shooting glue sticks in my 30 carbine or the speer capsules in my 45 colt] forward and that is when a fast pistol powder is used that will burn well at low pressure and rather quickly consuming all of the powder and spiking pressure in a rather abrupt up and down line.

and sometimes it isn't enough to move more than the powder around inside the case or just light it off.
this is when the neck tension becomes more of an influence on releasing the bullet after pressure has hit a certain point and doing so has more influence on accuracy and efficiency of the round.

now where the longer revolver rounds fall into place here is kind of a paradox as I can see them working both way's look at the 45 colt with a 165 rnfp and 5 grs of clay's or whatever the primer would be popping that bullet free and the powder would be spiking pressure rather quickly.
then switch that over to 2400 or H-110 and the tension and primer choice suddenly become more important in the outcome on paper since the primers blast would be negated by the powder fill the even neck tension holding things in place while pressure builds is more important.

 

[Rick]

Some things I haven't seen mentioned is age of the brass, how many firings, is it mixed with brass all of which has a different number of firings? All of which could be a cause of the primer moving the bullet before ignition. Neck tension and not the crimp is what will give consistent ignition.

 

[Paden]

Thinking out loud...

Seems logical to me that reduced air volume in the case due to higher density powder fill = less ability to absorb the pressure being generated by the primer detonation, thus more likelihood that the primer pressure will drive the bullet forward. Tho, the corollary might be that reduced pressure buffering capacity due to high charge density (and/or faster powder) will contribute to more rapid powder ignition, and thus faster and/or smoother pressure build / faster and/or more efficient acceleration at the beginning of the curve (all highly variable and dependent upon, but not limited to; case capacity, powder speed, bullet size/weight, etc., etc.).

Some things I take as givens:

Atmosphere is more compressible than solids.
Larger case capacity / more compressible air volume in the case = more ability to absorb pressure.
Powder charge (solids) displaces air / reduces capacity to absorb pressure.
smaller diameter / heavier projectile = increased resistance to thrust.
Larger diameter / lighter projectile = decreased resistance to thrust.
Skin friction (bearing surface area / neck tension / crimp) = increased resistance to thrust.

What I think happens (highly generalized / not all inclusive):

- Click: Firing pin strikes primer cup; crushes primer disk between cup and anvil.
- Pop: Primer disk detonates; generates a shock wave of pressure, sparks and flame into cartridge case.
- Pressure inside the cartridge spikes.
- Bullet responds to thrust, begins moving forward (degree variable: may or may not exit/unseal case mouth) / primer cup responds to thrust, moves rearward. (Between "pop" and this point is when most all of the crap is deposited in the primer pocket).
- Pressure begins to decrease as bullet moves forward.
- Powder begins to ignite; pressure builds again.
- Building pressure exerts equally in all directions, avenues of escape generate thrust: bullet moves forward; case moves rearward, re-seating the primer cup, and balloons against chamber walls; firearm begins to move rearward.
- Bullet accelerates as pressure continues to build; firearm continues to move rearward.
- Effective container volume increases as bullet moves down the barrel; pressure may decrease as a result, or may continue to increase...(highly variable depending on quantity and burn rate of the powder, and length of barrel).
- Bullet exits barrel; pressure released to atmosphere; sonic shock wave generated: *BANG*

The precise timing of how all these things meld together dictate: efficiency of acceleration / smoothness and speed of bullet engagement with lands / barrel harmonics and timing / shape of the pressure curve / felt recoil pulse / etc.

What am I missing, where am I wrong?

 

[fiver]

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.