Poll: What moves first, Bullet or Case?

Ominivision1

Ok, I have said bullet moves first (even if its thousands of an inch) before case expands at the neck to seal the gases to push bullet out of the muzzle. Whats your opinion?

gelandangan

The case is lighter BUT it it SHOULD be butt packed against the breech,
OTOH the heavier projectile only have its own momentum to overcame.

IMO in a good gun, it is the projectile that moves, and the case stays put.

fnuser

the primer will move first if you have ever seen an uncharged case fire the primer will be backed out slightly because the resulting charge expanding pushes the case back around the primer.

Elvis

the case has to move a little either forwards or expanding before the projectile.

minerfile

Newton says that for every action there is an equal and oposite reaction, so both should move at the same time, but then again fnuser may be right that the firing pin striking the primer may initiate the first action.

Vince

I'm going for the case.

If we are talking nanoseconds (NSEC), then as Miner mentioned...Newtons Law...the powder charge starts burning at the primer end of the case, with the resultant pressure expanding in all directions. The base of the case is the first "solid" object that pressure will encounter, followed by the walls of the case and ultimately the projectile.

As I said...nanoseconds, but because the pressure is expanding equally in all directions, the closest part of the cartridge to the point of ignition is the base of the case. Bear in mind that a NSEC is .000000001 (10-9) or 1 billionth of a second.

Cheers, Vince

Ominivision1

You bring up a good point fnuser, one that was not mentioned in "RIFLE ACCURACY FACTS By: Harold Vaughn". This is why I started this poll, to get other opinions. According to his book, he said that the neck would be completely disengaged (radial movement) before the bullet had moved.

But I'm still thinking the bullet does move (in thousands of inch) before the neck expands. The bullet moves when the force against the base (bullet) exceeds the sum of the friction force on the neck and the force the lands exert against the front of the bullet. If bullet is not seated into the lands then bullet moves when base force exceeds friction force of the neck. Popcorn

Suzanne

I can't see that the neck would expand, to the point of completely disengaging the bullet, before the bullet moves. The crimp and the bullet cause the build up of pressure to exceed the crimp's ability to hold the bullet. When the bullet moves, the crimp expands past the cannelure of the bullet. What happens when you over-crimp? Does the neck expand, disengaging the bullet? or does the pressure build to an intolerable amount before the bullet moves?

FN why would a primer in an uncharged case tend to move at all, if there's nothing in the case to cause pressure build up? Or do you mean uncharged, but with a bullet seated?

It obvious the case has to expand after a build up of pressure. The resulting pressure exceeds the crimp's ability to hold the bullet and so the bullet moves.

An everyone
knows I"m
an espert
Suz

fnuser

ok I probably didn't explain that very well but the firing pin strikes the primer driving everything foward especially if you have a chamber where the brass isn't fireformed to. If you do there still is some spring in the brass that is why it works and why you don't use empty brass for headspace guage. Now the primer encounters resistance and fires throwing the spark through the vent, (remember this another cavity connected by a tunnel when the primer fires it backs out a little.) and igniting the powder charge and the case is forced against the bolt face and the entire process of recoil is started. If there isn't any powder to start the recoil process the primer will back out of the case slightly and not be forced back in where we usually see it after extraction. We haven't even started to talk about the bullet leaving the case yet because in the world of nano-seconds this is still in the future.

Aloysius

Who was first, the egg or the chicken?

I don't know about you, but I'm not a benchrester and with my shaking, both case and bullet (and even powder and primer) are moving before the firing pin moves forward...
and when the internal PC puts it all together (movement of the target, the gun, the finger, the wind, ...) then the target has a problem. Small mistakes means work for the dogs. Big mistakes make a happy target. And who cares what's first or firster?

Ominivision1

Aloysius wrote:

Who was first, the egg or the chicken?

The zygote cell is the only place where DNA mutations could produce a new animal, and the zygote cell is housed in the chicken's egg. So, the egg must have come first.

Suzanne wrote:

What happens when you over-crimp? Does the neck expand, disengaging the bullet? or does the pressure build to an intolerable amount before the bullet moves?

The tighter the crimp, the more the pressure will rise to get the bullet moving.

Pumpkinslinger

I'd think that, if you are talking about movement due solely to the ignition of the powder, they’d both move at the same time in opposite directions. Newton’s Third Law, equal and opposite reaction, etc. If you mean movement due to the strike of the firing pin then again they both move at the same time, and in the same direction, because at that point they are still one unit.

Ominivision1

Actually, It all happens about the same time when you consider it all takes place within about .00001 second. However when you put numbers with all the variables and do the math you come up with an answer. That is, in a 6PPC with a 70 grain bullet, .001 neck interference, .01 neck thickness, .24 neck engagement, and .3 friction coefficient between bullet and neck the bullet would had moved about .000002 (yes I know it's not a lot) in when neck tension has dropped to zero. All this happens when the pressure has only risen a few hundred psi. Ok I'm Done.
Popcorn

slimjim

How to keep gun nuts busy all day!

Pumpkinslinger

slimjim wrote:

How to keep gun nuts busy all day!

Too true!