Measuring primer seating depth.

[918v]

OK, but what causes a floating diring pin to discharge a primer that is not fully seated vs. one that is fully seated? For example, if one were to seat a primer perfectly flush with the casehead and another slightly deeper. Why would the flush seated primer be more likely to discharge and not the bottomed-out primer. That proposition defies logic. Everyone knows that a primer which is fully seated to the bottom of the primer pocket is more reliable than one which is not. Why would a floating firing pin reverse that?

 

[GW Staar]

Speer explains it as good as anyone, and better than me. Quoting from the first page in the Speer article linked to in my first post: (read the whole thing)

Virtually all U.S. military Service Rifles utilize firing pins that rest freely within the bolt. Referred to as a floating firing pin, it will actually strike the primer lightly when the bolt is closed. This results in a slight dimple in the primer, which is plainly visible if the unfired round is extracted. This isn’t a problem with military ammunition because they use primers with thicker cups specifically because of this. However, it can be hazardous with the more sensitive commercial primers. The risk increases with high primers, headspace problems, and poor gun-handling technique. These risks are covered in greater detail elsewhere in this section. Please pay particular attention to the sections about rifle manipulation, sizing and priming.

Now you know why CCI started making "military" style primers to sell to the gas-gun shooting public.

BTW the whole article is in Speer's latest manual.

Everyone knows that a primer which is fully seated to the bottom of the primer pocket is more reliable than one which is not. Why would a floating firing pin reverse that?

Never said that. A floating firing pin doesn't reverse that. Fully seated, yet not crushed, will always be the goal.

 

[918v]

But Speer does not address the mechanics of the floating firing pin vs. the primer position within the primer pocket. Speer addresses primer sensitivity.

In post #20, DaveP alleges that a primer seated below flush with the casehead is less likely to slamfire in a floaring-firing pin system. I'd like to have that explained because I don't understand how.

 

[GW Staar]

But Speer does not address the mechanics of the floating firing pin vs. the primer position within the primer pocket. Speer addresses primer sensitivity.

In post #20, DaveP alleges that a primer seated below flush with the casehead is less likely to slamfire in a floaring-firing pin system. I'd like to have that explained because I don't understand how.

It's simply a matter of reach. Think boxing, except think of the glove traveling against more friction, say in thicker air or even water. Damage caused by the glove is a function of glove speed, and reach. If the glove meets your face at the start of the forward stroke, lack of speed reached will do little damage. When adaquate speed is reached first...major damage. Friction adds to the equation as is moves to the end of the arm's reach, slowing it down slightly, but even more important, the full weight behind the hit doesn't reach the target, so less damage. In the firing pin's case, the firing pin has a stop (its reach). If the primer is too far forward (towards the pin) the pin can drive through the primer (figuratively) with all its weight, unhindered by the firing pin's designed stopping point. The round can slam-fire.

 

[jfdavis58]

Unless you enjoy the tedium of collecting data and running statistical calculations, single seating depths tell you very little. In fact knowing the seating depth from measuring a head to primer distance probably tells you nothing.

Far and away the more important issue (and then likely only for rifles) is the pocket depth AND THAT IT BE SQUARE (shouting on and for a purpose). Safety issues demand that the primer be below flush. Uniformity of ignition says they need to be located at approximately the same below flush dimension plus or minus a very small variation.

With quality pistol brass a complete cleaning of the pocket is usually sufficient to put primers a couple thousandths below flush and allow a solid seating contact between primer and pocket.

Marking the case at several points around the head circumference; measuring the pocket depth at these points; collecting the data and reducing it to some average depth, and maximum depth; then cutting all to this maximum depth (assuming it's still in spec) is the process. Expect to cull some number of cases that cannot be made square in the pocket and still be kept within the nominal (optimal) depth maximum.

Removing more than the measured statistical maximum amount of material is counter productive, i.e. there is no need to go to the pocket depth maximum (unless you know you will use very tall primers--primers don't all have the same height dimension.) Finding good primer dimension data is required as well as finding nominal pocket dimensions.

I doubt the inertia of a firing pin traveling to some distance X is significantly greater than the same pin traveling to X plus 1 or 2 thousandth of an inch; use primers specific to your application (gun) The reasons to seat primers squarely (against a solid surface) is obvious and logical--better uniformity of ignition. Seating primers below flush is obviously a safety issue-primers are sensitive and we don't want to offend them with rough handling (in the box, loading to a magazine of any type or while chambering).

 

[918v]

It's simply a matter of reach...

In the firing pin's case, the firing pin has a stop (its reach). If the primer is too far forward (towards the pin) the pin can drive through the primer (figuratively) with all its weight, unhindered by the firing pin's designed stopping point. The round can slam-fire.

If the firing pin has a "stop" that prevents it from striking a properly seated primer with full force as the bolt closes, how does the hammer overcome that "stop" to discharge the primer?

 

[GW Staar]

If the firing pin has a "stop" that prevents it from striking a properly seated primer with full force as the bolt closes, how does the hammer overcome that "stop" to discharge the primer?

OK, I admit that stop was a stupid word choice, because before M1's and M14's, and M16's for that matter, were given "lighter" firing pins, even properly seated factory rounds were going off when they shouldn't. The M16 just about got canceled until they "fixed" it with lighter firing pins. IMO it must be that the added length of travel, (friction resisting the lighter firing pin) must slow the forward momentum of the lighter firing pin just enough to prevent a slam fire situation, at least with a properly "hard" primer cup, and a light enough firing pin.

Quoting from Speer again:

In semi-autos, this condition becomes downright hazardous. With their higher bolt speed and greater inertia, the chances of a high primer igniting when the bolt drives home is greatly increased. In the case of Service Rifles in particular, high primers are far and away the leading cause of slam-fires. Slam-fires are very rare in serviceable rifles using military ammunition. Handloaded ammunition, unfortunately, is another story. Most slam fires can be traced directly to certain errors in the reloading —and most often, the priming process.

Admittedly, they didn't (or maybe even couldn't) actually explain, beyond any doubt, the actual process of how it happens, but after reading their whole article, it's obvious to me that they've tested this scenario for years. And even if they can't guarantee the exact mechanism, it still happens. I'd rather not load high primers in my rifles to see it they're full of sh..t. You can if you want.