SigP320

[gc70]

Typically striker fired guns have some sort of "sear engagement" between a frame mounted component and a slide mounted striker. If there's significant play between the slide and frame, that play affects the amount of sear engagement.

Your very succinct description may provide a clue to the question.

As you noted, the relationship between the release mechanism and the striker is critical. That typically means little or no potential for vertical movement between the slide holding the striker and the frame holding the release mechanism.

Could the movement you noted be between the fire control unit holding the release mechanism and the frame rather than between the slide and release mechanism as typically found on striker-fired guns that are not modular?

 

[JohnKSa]

Could the movement you noted be between the fire control unit holding the release mechanism and the frame rather than between the slide and release mechanism...

I suppose it's possible but given the reports of persons commenting on the vertical play between the slide & frame the simplest explanation may apply.

I think that from a practical standpoint, play between the fire control unit and the frame would affect sear engagement in exactly the same way as vertical play in the slide and would pose the same potential safety issues. It might be easier/cheaper to remedy though, depending on how the gun is constructed.

 

[pblanc]

Well, I just put the first 100 rounds through my full-sized P320 .45 ACP and I watched very carefully for any upward slide movement while shooting. No hint of it and the gun is a delight to shoot. It absorbs recoil remarkably well.

 

[gc70]

I think that from a practical standpoint, play between the fire control unit and the frame would affect sear engagement in exactly the same way as vertical play in the slide and would pose the same potential safety issues.

I don't see the relationship between the grip module (frame) and fire control unit having an effect on the amount of play between the rails on the slide and fire control unit. The play in the rails is what allows movement that affects sear engagement.

At any rate, I am back home and will measure the movement on my P320 with the fire control unit in three different grip modules to see if there are any differences.

 

[gc70]

Back home and with access to my P320, three grip modules, and a set of feeler gauges, I measured the distance between the back of the slide and each grip module ("frame"): with the striker cocked and released and, for each of those conditions, with and without a magazine in the pistol. The results were:

#1: (mag) released 0.010" - cocked 0.004" (no mag) released 0.009" - cocked 0.003"
#2: (mag) released 0.010" - cocked 0.004" (no mag) released 0.009" - cocked 0.003"
#3: (mag) released 0.009" - cocked 0.003" (no mag) released 0.008" - cocked 0.002"

In all cases, there was 0.006" of movement between the striker being cocked and released.
In all cases, the presence of a mag pushed the slide and FCU up 0.001" in the frame.
One grip module positioned the slide and FCU 0.001" lower in the frame than the others.

When the striker is cocked, positive sear engagement appears to pull the slide down toward the FCU within the tolerances of the rails. When the striker is released, the slides move up from the FCU within the tolerances of the rails.

I spoke to my son, who is a Glock Advanced Armorer, and he suggested the slide movement would be visible when dry firing, but recoil would occur before the movement was noticeable during live fire.

Whether the movement between the slide and FCU could present a hazard of the striker being bumped out of engagement with the sear is shown in the attached photo. I placed a 0.006" feeler gauge (the amount of movement) next to the striker's sear engagement surface; a 0.006" movement would be a small fraction of the striker's sear engagement surface.

 

[JohnKSa]

The play in the rails is what allows movement that affects sear engagement.

Sear engagement is affected by any play between the "sear" and the striker. That play can be caused by vertical movement of the slide relative to the frame, relative to the fire control unit, or maybe relative only to the sear, depending on how the gun is constructed and where the play exists.

In other words, even if there were no play at all between the slide and the rails, if, for some reason, there were play between the slide and fire control unit (e.g. the fire control unit fits loosely to the unit which incorporates the rails) that would still affect sear engagement.

To be clear, I don't think that's what's happening here, I was just responding to your question about the possibility of "movement ... between the fire control unit ... and the frame".

... recoil would occur before the movement was noticeable during live fire.

I agree. There's no way this would be noticed during live fire.

I measured the distance between the back of the slide and each grip module ("frame"): with the striker cocked and released and, for each of those conditions, with and without a magazine in the pistol.

Interesting results. 0.006" is not a lot of play.

https://gastatic.com/blog/wp-content/uploads/2015/02/Sig-P320-internal-safety.png

From the picture in the link, it appears that the sear and striker engagement surface are slightly slanted which means that when they are engaged, there's a downward force on the striker (and anything it's attached to). It also means that they are unlikely to come out of engagement with each other once they're engaged.

I placed a 0.006" feeler gauge (the amount of movement) next to the striker's sear engagement surface; a 0.006" movement would be a small fraction of the striker's sear engagement surface.

I doubt that the full surface is actually engaged. In the Glock design, for example, the actual engagement is usually only around 70% of the total possible.

 

[gc70]

Sear engagement is affected by any play between the "sear" and the striker. That play can be caused by vertical movement of the slide relative to the frame, relative to the fire control unit, or maybe relative only to the sear, depending on how the gun is constructed and where the play exists.

Another excellent general description. With respect to the P320, the frame (grip module) can be removed from the equation regarding striker engagement because the fire control unit (FCU) and slide are operationally self-contained - even with the frame entirely removed as shown in the first photo posted above. There is also no discernible vertical movement of the sear in relation to the FCU chassis. The tolerance between the FCU chassis rails and the slide rails appears to be the only mechanism affecting sear engagement on the P320.

To be clear, I don't think that's what's happening here, I was just responding to your question about the possibility of "movement ... between the fire control unit ... and the frame".

I assumed movement between the frame and FCU chassis might be involved in the upward movement of the slide. I was wrong. Beyond the 0.001" difference in slide-to-frame gap when a magazine is present versus when one is not, I was not able to detect any contribution to slide movement.

From the picture in the link, it appears that the sear and striker engagement surface are slightly slanted which means that when they are engaged, there's a downward force on the striker (and anything it's attached to). It also means that they are unlikely to come out of engagement with each other once they're engaged.

The surfaces are slightly slanted. The infinitesimally small rearward movement of the striker needed to overcome that positive sear engagement has been pointed to in other discussions as the probable basis for SIG's claim that the P320 has a double-action trigger (even if the striker is only "cocked" a percent or so as the sear rotates downward).

I doubt that the full surface is actually engaged. In the Glock design, for example, the actual engagement is usually only around 70% of the total possible.

On my Shield, there is an obvious wear line from the sear roughly across the middle of the striker's engagement surface. My P320 has no such wear mark, so I don't know how much of the surface is actually engaged. I suppose I need to shoot my P320 more until it develops a wear mark.

 

[JohnKSa]

The surfaces are slightly slanted.

That accounts for the slide jump. In the Glock design, for example, the engagement surfaces are essentially vertical. That means that there's no downward force on the striker when the "sear" and striker are in engagement. Since there's nothing pulling down on the slide, there's no reason for the slide to move upward when the striker is released even if there's play in the slide.

In the P320 design, the slanted engagement puts some downward force on the striker/slide which means that if there's any significant play in the slide, when the downward force is released the slide may jump upwards.

The slanted engagement also suggests that the sear/striker are less likely to come out of engagement than in a design with a vertical engagement surface. Seeing how the design works makes me less concerned about safety issues and sort of pushes the "jump" more into the realm of aesthetics.