let's break this down.

[fiver]

Brad prompted me to talk about the firing sequence of a cartridge.
so i'll start it out as a discussion of each sequence when pulling the trigger.
this should help us visualize what we are manipulation when we change things in a load.

the first thing we should discuss is primer ignition and the brisance level of the primer and what it does to the powder and the case itself.
next it should be powder burn rate.
then pressure on the case.
then how the bullet can influence both the burn rate and pressure.
we should also discuss how the case swells to fill the chamber.
and finally how case shape influences pressure/burn rate/and gas movement within the case.

lets talk about the primer.
we know that different primers have different brisance levels, but what exactly does this mean [and mean to you]
what exactly does the primer do to the powder when it's lit off?
and why would a change from one formulation to another make a difference on target.

 

[Brad]

Number one key with primers is uniformity of igniting powder. A "match" primer isn't always the answer as it may not ignite the powder in the most uniform manner.
Match the primer to the case capacity and powder. I can see where a mag, or "hotter" primer might help with heavy charges of really slow ball powders like 870.
Primers are something I view as a fine tuning tool. I don't expect major changes due to primers but I can see where poor ignition could be fixed some by primer changes.

Primer brisance is like bullet hardness, more isn't always better. It is more a case of finding the right level for the specific load.

 

[fiver]

okay we hit that primer with the firing pin.
but what then?
does the primer cup absorb the blow or does it allow the case to be driven forward in the chamber.
both is my answer how much of both depends on case fitment and the cups hardness.
it also sets up a little sine-wave resonating down the barrel ahhh.
the next thing that happens is it pretty much explodes/ignites and sends a shower of sparks/flame into the powder column.
ooooh so which is it a shower of spark or a flame?
yes.
what kind of answer is that?
well it's kind of both depending again on the primers formulation, the sparks do come out but the primer compound will burn and shoot out a flame front insuring the powder ignites and burns.

 

[fiver]

mag primer doesn't always mean magnum charge.
I sr mag primers in my 25-20/30 carbine/and other small cases.
most magnum primers do one of two things they either burn hotter or longer.
remember the old Winchester primers that were marked standard or magnum they done that by burning longer not hotter.

 

[Ian]

I think some or most magnum primers also have a harder cup. Outpost75 put up one heck of a post somewhere in the past month or so talking about primer brands, compositions, how they differ, and why it makes a difference. I recall something about aluminum powder in some as a fuel that sprayed molten metal into the powder charge. Very interesting and pertinent here, wish I could remember where that post was.

Wanna throw off a very delicately-balanced load? Push that shoulder just five thousandths back too far in the bump die. I explained in one of the basement articles about using the datum to pre-load the bolt lugs and how doing so affects barrel harmonics, seen it happen a lot, which is why I'm so careful about just how much I bump shoulders in bolt guns. But I'm getting ahead here.

Something else that affects the primer's "net" brisance in a given application is if the powder is loose in the case or held firm by some manner of slight compression (case full of powder or a top-off with buffer or a little Dacron holding the charge against the flash hole). Like the little tuft on top of 45 grains of H414 bumping the burn rate a click.

 

[fiver]

aluminum sphagnate in federal.
and lead styphnate in Winchester.
I dunno about rem and cci, and have never really looked since I rarely use them outside of a couple of specialty loads.
I do know cci-300's would be my primer of choice in straight walled hand gun cases with flake powders if I had to pick just one specific application.

 

[Ian]

Three Smith & Wesson revolvers that can shoot well enough to tell me all agree that they prefer Federal standard primers...even with 2400 powder. Probably has to do with the weak-assed hammer springs. CCI-300 is what gets bought and stocked in bulk quantity, though, they're just dead-on consistent year after year and I know I can count on them.

 

[Brad]

It is easy to see that you really can't look at just one component alone. The primer, and it's actions, are work in concert with many other factors. That is part of what can make all of this confusing at times.
When we change primers the primer output may not be what made the change, it could just be the strength of the cup in compairison to firing pin impact.

I need to go back and do some looking thru old magazines. Seems I read something somewhere that said a primer that just barely works is the best in many cases.

 

[fiver]

well it could be the cup it could also be the make-up of the primer hitting the kernals/flakes/balls of powder.
in my testing I have found that the federal 210 primer does best with stick powders like 4831 and rl-19.
sometimes the Remington 9-1/2 does better with them.
the newer win non plated primer is better than the old plated one but still isn't the better choice, it however does better with the more dense powders like cfe-223 and 748.

I know we have all seen those pictures of primer sparks flying out of a cut down barrel, think about those sparks..
they are just the initial force going into the powder they are not what really lights it off.
they are more a disruptive force inside the case trying and succeeding at moving things around in there.

probably breaking some of the kernels down.

I know if you use a gentler primer and don't add more powder back into the case to gain back that little bit of lost velocity it will for sure affect accuracy.
wait... add more powder?
how do you get the same velocity/pressure with more powder?

anybody...

 

[yodogsandman]

The gentler primer wouldn't light off some powder initially that would be lit by a more aggressive primer, reducing velocity. Powder burned while still in the case and before bullet movement causes more force from expanding gasses than powder burnt while the bullet is traveling in the bore.

 

[fiver]

did I hear slower more consistent pressure rise.
hmmm I think that can be helpful in getting my boolit moving forward where I can then see the real engraving spike and start the pressure climb in the right place.
[after my boolit is halfway or more into the barrel]
without a bunch of turbulence going on behind it.

 

[Ian]

With loads in the jacketed starting to full-velocity powder charge range, pressure peaks within the first couple inches of bullet movement, even with powders like 4350 and 414. That, however, is a huge improvement from peaking the pressure while the bullet is still in the throat.

I don't know exactly what happens inside the case, only what changing some things slightly does to muzzle velocity and POI. Basically, primer/powder interaction affects everything about the total pressure curve, but how exactly, I couldn't say.

Here's an analogy that I use a lot in my own head when thinking of getting the powder lit and the resulting initial powder burn up through the point that the bullet just starts to move: Drag racing on the strip. You have to have the RPM (initial power input), gearing, brake application, etc. just right to maximize the launch off the line. If (for the track and tire condition) you put too much power or apply the power too quickly for the first few milliseconds, you'll break loose the tires and lose traction at the launch. If you don't put enough power or apply it too slowly you lose acceleration time. The engine has the same potential horsepower and torque either way, it's how you apply that power and when, to match the resistance of the load, or how you manipulate the load (burnout time, tire heat, application of traction compound, rubber composition, air pressure, gearing, etc. etc.) that delivers the most controlled, best launch.

 

[fiver]

and that's exactly the start of a primers ignition sequence.
think about how a filler makes velocity variations suddenly stop and your es's settle down to more like 15 fps.
your holding everything in place allowing the primer to do the same thing the same way every single time.

a little off topic here but adding in a kicker charge to a too slow powder helps the same way.
you mitigate the erratic burn of the powder by getting it to burn consistently [acting like the primer is burning hotter and longer] and getting it into a slightly higher pressure zone sooner so the powder has a chance to consume itself before the boolit is too far away.

 

[Brad]

Is this why the BPCR guys get such low ES's? They use a compressed charge of powder with no empty space. Powder can't help but burn in a consistent manner. Same thing as a full load of H110 in a revolver. Powder is suddenly in an optimum situation to ignite and burn the same way each time.

 

[Paden]

okay we hit that primer with the firing pin.
but what then?
does the primer cup absorb the blow or does it allow the case to be driven forward in the chamber.
both is my answer how much of both depends on case fitment and the cups hardness.
it also sets up a little sine-wave resonating down the barrel ahhh.
the next thing that happens is it pretty much explodes/ignites and sends a shower of sparks/flame into the powder column.
ooooh so which is it a shower of spark or a flame?
yes.
what kind of answer is that?
well it's kind of both depending again on the primers formulation, the sparks do come out but the primer compound will burn and shoot out a flame front insuring the powder ignites and burns.

...And while sending that little sine-wave down the barrel and the sparks/flame into the case, it's generating pressure and starting to move the bullet forward out of the case.... Yes? How far does that bullet get before the powder charge starts to burn and build additional pressure behind it...and what are the implications thereof?

 

[fiver]

right Brad if you look at a recipe for a good black powder load they will specify compression amount and either filler cards of a certain thickness or a depth/height you want the powder column at before seating the bullet.

Paden.
IMO
the primer shouldn't move the bullet forward, the neck tension should hold everything tight.
I know we have heard of primers pushing the bullet into the cylinder gap etc.
but should they really do that?
or was it an under charge of powder.
the entire gun moving rearward when another round is fired doesn't do that [and it isn't all crimp doing the work]
in a revolver the bullet would fly forward into the barrel and the powder would just throw a bunch of flames out the cylinder gap.
if it didn't make it that far it would hit the cylinder throats and then the powder would for sure spike like crazy and you would have a SEE event.
a consistent even neck tension is important to good quality ammunition it cannot be there to just hold the primer blast back.

 

[Paden]

IMO
the primer shouldn't move the bullet forward, the neck tension should hold everything tight.
I know we have heard of primers pushing the bullet into the cylinder gap etc.
but should they really do that?
or was it an under charge of powder.

Is it a question of what we would like to hope happens, or of what actually happens? I'm loath to run outside and test it, just to say I did, as I'm pretty confident I'll be driving bullets out of barrels as a result...

 

[Ian]

Most of the time a primer alone will move the bullet until the bullet encounters higher resistance, like in the throat. Not always, though. I loaded a .30-'06 case with corn meal and the primer didn't budge the bullet. I'm still pretty sure it's powder gas pressure pushing between the kernels that moves the bullet in a bottlenecked rifle cartridge loaded with real rifle powder to a high density.

 

[Paden]

Yup, I was right... a GM210M primer moves a 347 grain cast bullet forward from an empty .45 Colt cartridge enough to tie up a revolver. (About 0.140 inches movement).

 

[fiver]

well Winchester standard primers drove 3-165gr 45 acp boolits cast boolits [averaged out] .220 deep into the rifling.
this was with 2 different seating depths.
this was basically the base driven into the rifling and just clear of the case enough to hear a pook.
good news is the boolits sealed the bore and there was no gas cutting.