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Old April 5, 2012, 09:10 AM   #19
Bart B.
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Join Date: February 15, 2009
Posts: 8,927
dmazur, your comment:
Quote:
It can affect primer ignition as well, if the case is not held tightly against the bolt face by an extractor. If the case head can't be reached by the firing pin, the cartridge may not fire.
This is another popular belief. And again, 'tain't what happens. Here's why.

All firearms' extractors lip distance from the bolt face is greater than the case rim's thickness; by a few to several thousandths of an inch and none of them pull the case head back against the bolt face (at least as far as I know).

Firing pins protrude a few dozen thousandths of an inch from the bolt face when in their full-forward position. Typical rifle pin protrusion is .055" to .060". More than enough to make up for a huge amount of head clearance or space between the bolt face and case head where the primer's virtually flush with.

When the firing pin strikes the primer, the case moves as far forward in the chamber that its "head clearance" allows and then stops, the primer doesn't fire until the cup gets dimpled next by the firing pin and crushes the priming pellet such that it explodes/detonates/burns-fast (whatever you prefer to use).

A good way to see this happening is use several reloaded rimless bottleneck cases each one having 1% less powder than the first one with a maximum powder charge. Fire those reloads in order, max load first down to the last one that's loaded with a powder charge 10% to 15% below max. Measure each fired case headspace with a gage in order. At about 5% below max, you'll notice that fired case headspace starts getting shorter and primers are backed out a bit. I've done this with a .30-06 case and at 10% to 12% below max, fired case headspace is .010" less than where it started at and the primer's backed out about .010". To me, that's proof that the firing pin pushes a rimless bottleneck case hard enough into the chamber that its shoulder gets set back before the round fires. And with loads reduced enough, there's not enough peak pressure to expand the case fully back until its head stops against the bolt face.

Note that for rifles with a spring-loaded plunger, in-line ejector in the bolt face, that pushes the case forward until it stops against the chamber headspace point; there's a gap or space between the bolt face and case head. Extractors have enough clearance to let this happen. Besides, there has to be enough clearance between the bolt face and extractor lip to let the extractor slide easily over the case rim's lip when chambering the round.

Last edited by Bart B.; April 5, 2012 at 09:16 AM.
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