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Old November 6, 2014, 03:51 PM   #1
Magnum Wheel Man
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what is the burn profile of ignition from a primer???

the live primer discussion, part way through the discussion turned to comparing rim fire primers, & popping them off without powder & bullet...

this coupled with the pictures of the cut off cases in the thread discussing uniforming flash holes a week ago, got me thinking...

how does the burn profile effect a cartridges performance???

going back to the rimfire cartridge, priming compound encircles the rim, however the hammer only strikes a small area of the rim, igniting the priming compound in that small area... does this small area light the powder, or does the priming compound burn 1st, giving a "ring of fire" igniting the powder???

on a center fire, going back to the pictures using a Lyman flash hole uniforming tool, & the sectioned cases, in trying to eliminate all of the burr, the poster ended up coning the flash hole on the inside... would this profile improve the burn characteristics at all, ( I use a lyman uniforming tool, & there is a taper on the cutter, rather than a straight cutting edge ) or if one could machine the burr off cleanly with a straight edge to the flash hole, would that be better??? I'm trying to figure out why Lyman would shape the cutter that way ???

& then is there any merit to "detonation" if the "blast" from a centerfire primer misses the powder on a lightly charged case, hits the base of the bullet, & bounces back to the powder charge, causing it to burn from the case mouth to the base of the case ( in effect, backwards )... or is detonation more likely the interpretation of the bullet getting popped out of the case, & entering the restriction of the rifling, before the charge ignites, weather from slow burning powder & light crimp, or ??? & causing the pressure to spike higher than normal for the said charge... & if this is the case, isn't it just as likely to have a huge pressure spike by loading long, & seating the bullet into the lands of the barrel ???
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Last edited by Magnum Wheel Man; November 6, 2014 at 03:59 PM.
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Old November 6, 2014, 04:27 PM   #2
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....and then there are Berdan primed!
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Old November 6, 2014, 04:30 PM   #3
Magnum Wheel Man
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^^^ Exactly...

I swear I saw some Berdan primed cases the other day, that had 4 holes in them instead of the normal 2 holes ???
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Old November 6, 2014, 05:02 PM   #4
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Someone had some images years ago (maybe it was in an issue of Precision Shooting) of cut off cases like yours fired in the dark with the same primer in each one but different degrees and shapes of burrs on the inner sides of the flash holes. The flame shapes did differ. IIRC, the deburred flash holes produced a consistent looking wider and shorter flame 'bush' of sorts, while the undeburred flash holes had some flames long and narrow, some wide, some fanned out, some diverted off to the side, and other variations.

Exactly how that affects powder is a little more mysterious. There were no grains in the way of the flames in those images.

As to detonation, it happens so very infrequently that it is not predictably reproducible thus far (AFAIK). I know that detonation of gasoline and air mix in engines was once thought to be caused by the sort of reflecting pressure waves you describe, but ultimately that proved not to be true. Using transparent ceramic cylinders and super high speed photography, they found the detonations, when they occurred (knocking in an engine) invariably originated at the hottest point in the cylinder. I expect that will turn out to be true in powder if they figure out how to make it happen consistently.

My theory: Since detonation is burning by shock wave ignition at or above the speed of sound in the explosive, some portion of the powder has to be fused by heat first for detonation to occur. Most of us have see fused powder from squibbed out loads at one time or another. So there is heat, probably from initial combustion, that, in the absence of being able to raise pressure rapidly in the large empty case volume, instead fuses most of the powder before finishing burning it. Once the powder is fused and still very hot and burning at the surface, the flame may initiate detonation the same way it dies in a knocking engine or in warm nitroglycerin. But you have to have a lot of melting energy in the powder first so that it is 'anxious' to burn and is right on the edge of doing so. Then what starts out as deflagration can graduate to detonation by making localized gas pressure rapidly enough to act as its own detonator.

Overall pressure in the chamber during such an event would not be high, owing to the small volume of gas generated by the small charge. That's what allows melting time. But, once detonated, pressure right at the fused mass could momentarily and only local to the powder mass, approach what instantaneous release of all the energy in the mass would provide, which is around half a million psi. That's enough to make a tiny crack in the steel. Once a crack is started, just a few hundred psi is enough to spread it and open the gun up. So I'm expecting a true detonation will have at least one small site where the steel is shattered at the inner chamber surface rather than torn open. The rest would be torn open and look just like an overcharge burst does.
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Old November 6, 2014, 08:46 PM   #5
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It's pretty well known that defomed flash hole's inside profiles will change the primer flash from shot to shot. How much it impacts ammo performance is often debated.

There's a number of folks who produce the same accuracy levels with unprepped flash holes as those who meticulously make them as perfect and uniform as possible. Sierra Bullets does no case prep whatsoever on cases used to test their best match bullets. Yet they shoot into sub 1/4 MOA groups in their 200-yard indoor range. In their California plant's 100 yard range, they would shoot around 1/8 MOA. Several people shooting long range matches don't prep their primer flash holes and produce results as good as those who do. I've not seen any difference in my tests at 600 and 1000 yards. That may be the differences in how people measures accuracy; I use extreme spread of many shots while others may use the smallest few-shot group. There is no standard as far as I know.

A few decades ago, a few people put flash tubes in their .308 Win cases to ignite powder from the front like several large caliber military case primers are. Tubes about 1.2" long the diameter of and threaded as a 6-48 screw with a .080" hole were threaded into flash hole reamed out and tapped to receiver them. Accuracy was as good as with normal ignition at the back end of the case. And it was no easy job to put a 1.5" long decapping pin down those tubes to push the fired primer out.

Flash hole deburring and uniforming is, on my list, the last thing to do for best accuracy. It's a waste of time until you shoot your stuff no worse than 1/3 MOA at the shorter ranges. And you may wear out a barrel shooting enough groups to prove the difference.

Last edited by Bart B.; November 7, 2014 at 07:54 AM.
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Old November 7, 2014, 12:50 AM   #6
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Quote:
I'm trying to figure out why Lyman would shape the cutter that way ???
It's shaped that way because it's only necessary to cut the EDGE of the flash hole to remove the burrs

Cutting it so deeply it becomes cone shaped is TOO deep
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Old November 7, 2014, 09:55 AM   #7
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I do believe flash hole burrs will deflect the flash path as Unclenick mentioned in the Precision Shooting article. Ignition in engines are of similar patterns. The fuel chamber has next to no space and it is compressed. Traditionally one sparkplug was used today we have sparkplugs with 2 spark sources and some combustion chambers have 2 spark plugs. The experimentation continues with increasing compression and controlling spark... just like the cases.

With that stead the path (control) of the fuel (powder) ignition would influence the outcome. Through much experimentation would be required to learn just how the primer flash profile reacts. But I suspect more testing would require some fancy equipment and details. Bart B. you mentioned how Sierra had not found a issue but did you have all the details. Some thing tells me there is more to it. I am not being sarcastic but there is room for some loose ends.

That's what I think
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Old November 7, 2014, 10:25 AM   #8
Bart B.
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Quote:
Bart B. you mentioned how Sierra had not found a issue but did you have all the details?
I think I do. What are all the details you want to know?

They first came from the two people who worked in their plant reloading and testing all the ammo used testing their stuff for accuracy. I've shot many a match with the first one. And I've walked through their plant and observed all the stuff they do and use.

Sierra's been using unprepped cases testing their bullets for accuracy for decades. Perhaps the fact that they full length size all their cases in over-the-counter dies, use rail guns (which eliminates virtually all the other rifle and human variables effecting accuracy) with match grade barrels with virtual SAAMI spec chambers (not "tight" so-called match chambers), shoot in indoor ranges and don't work up new loads when changing any component lot. . . .has no bearing on the primer flash hole issue. All of which is their fancy equipment and details. It's no different that what benchresters use.

Sierra's tests are typically 10-shot groups fired with bullets grabbed as they come out of the final jacket forming dies (at 80 to 90 per minute). They're seated in sized and primed cases with metered (not weighed) powder charges. Then all 10 shot in the rail gun in a couple of minutes, measured, logged then 10 more are taken and shot. Several dozen test groups are shot during the production run of a given lot. I've seen a set of groups shot in the late '60's with their .308 168-gr. match bullets and all well under 1/4 inch/MOA with an average of just over 1/8 inch. The curent IBS benchrest aggregate record for five 10-shot groups at 100 yards is .1898 inch/MOA average and was shot with totally prepped cases including flash holes.

Complete match rifles have been tested with unprepped flash holes in the cases that shoot under 1/4 MOA at 600 yards and 1/2 MOA at 1000 yards; group after group after group. One tested some years ago (with unprepped flash holes) shot several 10-shot groups at 600 yards; all were under 1.5 inches. The NBRSA 600-yard aggregate records for six 5- or 10-shot groups are in the 2 to 3 inch range shot with totally prepped case flash holes..

I well understand the premise that uniforming every single element and detail of a cartridge will make it shoot bullets more consistantly. But when one gets down to those that make a low, single-digit percent change, can you shoot your stuff accurate enough to reveal it? On the other hand, seeing something "perfect" in your components does make one feel good. And nobody shoots well if they don't feel good.

Last edited by Bart B.; November 7, 2014 at 11:53 AM.
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