Hot 9mm load causes malfunction? Mechanical issue?

[drband]

I'm posting in reloading (EDIT: Just moved to gunsmithing and repairs--seems more appropriate now) because it seemed the logical place. Mods, please move if a better forum fits.

I carry a Sig P938 exclusively. Great shooter, very controllable, very accurate.

I recently switched from RMR 124gr RNFP to the new RMR 124gr FMJ for my practice ammo. I have observed this issue in the RNFP, but found it again in the FMJ (and the solution) as I was working up the new load for the FMJ.

Basically, the firing pin in the P938 will sometimes lock up when shooting near the max load of HP38/W231. The hammer will drop and I get a metallic click as it hits the locked FP (sounds and feels quite different from dry firing with or without a snap cap or empty cartridge in the chamber). The unfired case has a very tiny mark where the FP barely touched it. It happens randomly, but at least once per mag when shooting the hot load near max.

Worked up as follows:
9mm
124gr RMR FMJ
HP38/W231

4.0gr light load no issues
4.2 Better no issues
4.4 Very accurate, good load
4.6. More recoil but controllable
4.8 Hot, controllable FP locked 2X

Any ideas why this might happen? As long as I stay below 4.8gr the problem does not exist--ever.

Commercial loads (Aguila 124gr FMJ, Blazer 115 aluminum case FMJ, Federal 124 HST, and Speer 124 Gold Dot standard) do not cause the issue.

Any ideas... anybody ever seen anything like this?

Addendum: none of these loads cause issues in my RIA 9mm 1911, BHP, or Hi-point carbine.

 

[ArchAngelCD]

I'm at a loss as to how a different powder charge can effect a gun if the round does not fire. The gun can't know if you add 0.2gr more or less powder inside the case.

Usually when you get a light hit on a primer it's due to the primer not being seated completely. I know you said it's only at the top end loads but like I said, the amount of powder in a case that does not fire can't effect the gun.

 

[drband]

That's exactly why I'm mystified. I guarantee that the primer is seated fully. Seriously.
S&B sp primers.

At one point, I sent a previous p938 back to sig with this issue. It malfunctioned in the same way with Speer gold dots as well as with reloads.

I'm not sending this one back because it works perfectly with Federal HST, Speer gold dots, and with my reloads under 4.8gr W231. I just wonder if anyone else has seen something similar to this issue that could shed some light on the cause.

 

[rsrocket1]

Are you plunk testing every round as you are loading them? You're sure the powder is not being compressed and keeping the bullet from being fully seated? Maybe the ogive is a bit further out and getting interference with either the throat or rifling.
Light off center primer strikes can be the result of a slide that is ~99% closed. The strike may be enough to make the slide look fully closed but unless you check that the slide is fully closed before pulling the trigger each time (impractical), you'll never catch it.

 

[drband]

the amount of powder in a case that does not fire can't effect the gun.

I agree, but wonder if higher slide velocity with max loads somehow affects the FP/spring or drop safety/disconnector interface. That could be affected by changes in powder charge.
Recoil spring strength, too. There are a lot of variables to consider in these small pistols.

The previous p938 eventually locked up the FP/drop safety block under the same load with the other bullet. FP would not move.

 

[Blue68f100]

Wondering if your going into full battery or not. It may be the slide is not completely forward and internal safety (disconnector) is stopping the gun from firing.

Have you checked to see if your OAL is changing during fire?

 

[drband]

Plunk testing works 100% with all rounds. I did check--and reduced COAL a bit just to be sure before this work up. All the previous reloads plunked perfectly, too.

 

[drband]

Wondering if your going into full battery or not. It may be the slide is not completely forward and internal safety (disconnector) is stopping the gun from firing.

Have you checked to see if your OAL is changing during fire?

OAL does not change. Checked.

If the disconnector is in play, wouldn't the hammer fail to drop?

Or just engage the FP block?

 

[ArchAngelCD]

I know you are calling the 4.8gr W231 load a hot load but that is only the max load on the Hodgdon site with a 125gr Sierra FMJ bullet. It may be max but not "hot" by any means. Unless you are seating the bullet deeper that you would with a COAL of 1.090" your pressures should not be anywhere near max. Hodgkin is listing less than 28,800 CUP.

You are correct, this is a very strange problem.

 

[ReloaderFred]

Have you actually measured the depth you're seating the primers? They should be .004" below flush to be properly seated. I'm not sure how this could affect only the higher end loads, but the light firing pin marks are usually an indication of primers not being seated completely. The S&B primers are shaped flatter than domestic primers, and the anvil is pre-seated into the priming pellet, so it's imperative that the primers be seated all the way to the bottom of the primer pocket so they don't move under the impact of the firing pin strike.

Hope this helps.

Fred

 

[drband]

I probably should have said max, you are correct as Hogdon data seems conservative to me.

My COAL is 1.140, previously 1.150, both which plunk perfectly.

The Aguila is 1.130 which is why I reduced mine before work up this time. All these loads plunk and spin freely.

 

[drband]

Have you actually measured the depth you're seating the primers? They should be .004" below flush to be properly seated. I'm not sure how this could affect only the higher end loads, but the light firing pin marks are usually an indication of primers not being seated completely. The S&B primers are shaped flatter than domestic primers, and the anvil is pre-seated into the priming pellet, so it's imperative that the primers be seated all the way to the bottom of the primer pocket so they don't move under the impact of the firing pin strike.

Hope this helps.

Fred

I prime on press with my LCT and push the press arm quite firmly so I can feel the primer hit bottom. I guess it is possible, though.

I do make sure all primers are seated below flush of course.

Still I wonder why only on the 4.8gr load?

And... I can feel the hammer hit a locked FP. It feels and sounds different than a dry fire.

 

[243winxb]

The firing pin safety block may need adjustment? . Thats my guess after doing some research. http://sigtalk.com/p238-p938-pistol/5258-sig-p938-safety-ccw-questions.html

I assume you have the manual.
on page 12,

2.1.2 Principal Features
The single action only trigger, in combination with the thumb
safety, ensures safe carrying of the weapon and provides
instant readiness when needed.
The automatic firing pin safety blocks the firing pin unless
the trigger is pulled.
The hammer safety intercept notch prevents the hammer
from contacting the firing pin unless the trigger is pulled.
The disconnector prevents the hammer from falling when
the slide is out of battery (not fully closed).

 

[ArchAngelCD]

I probably should have said max, you are correct as Hogdon data seems conservative to me.

My COAL is 1.140, previously 1.150, both which plunk perfectly.

The Aguila is 1.130 which is why I reduced mine before work up this time. All these loads plunk and spin freely.

Yeah, your COAL is probably producing normal middle of the crowd pressures. Like I said in Post #2 and by ReloaderFred, it sounds like you are experiencing a primer seating problem. Next time it happens see if all the fail to fire cases are the same brand. Check all your primers before you shoot the next batch.

 

[drband]

Yeah, your COAL is probably producing normal middle of the crowd pressures. Like I said in Post #2 and by ReloaderFred, it sounds like you are experiencing a primer seating problem. Next time it happens see if all the fail to fire cases are the same brand. Check all your primers before you shoot the next batch.

Will do!

 

[dgod]

Is there a chance you have a few different head cases in the mix? Just a thought

 

[243winxb]

If you have had a pierced primer in the past, the metal disk may be in the firing pin channel. This disk may slow the firing pin speed on firing.

But i would guess, the amount of powder, high pressure would have no effect?

The simple fix is, reduce the powder charge, as you may be over pressure for that bullet.

 

[drband]

Under heavy recoil, you finger is coming off the trigger. This activates the firing pin safety block. Thats my guess after doing some research. http://sigtalk.com/p238-p938-pistol/5258-sig-p938-safety-ccw-questions.html

No, in slow fire at the range, I pull through and release after the cycle so I can feel the trigger reset. That's not it.
I don't think I could release the trigger quick enough to engage the DS anyway.
On this pistol the ejector keeps the DS block depressed/engaged until the trigger is pulled, rotating the ejector forward and down, releasing the block and freeing the FP.
When the malfunction occurs it feels like the block does not move to release the FP. You can feel the hammer hit a locked FP making a "tink" sound. Feels and sounds much different than a dry fire. Only happens at the 4.8gr load.

 

[drband]

Is there a chance you have a few different head cases in the mix? Just a thought

Yes

 

[243winxb]

See above, i change my answer. The series 80 Colt had 3 different size activators for there firing pin block.

 

[drband]

If you have had a pierced primer in the past, the metal disk may be in the firing pin channel. This disk may slow the firing pin speed on firing.

But i would guess, the amount of powder, high pressure would have no effect?

The simple fix is, reduce the powder charge, as you may be over pressure for that bullet.

No pierced primer. Pressures within safe range. However, your suggestion of a fix does work and my best load is at 4.4gr.
I just want to know why!

 

[dgod]

Is there a chance you have a few different head cases in the mix? Just a thought

Is there any deviation for the base of the bullets causing the issue? Coud it be as simple as a few pieces of brass being out of SAAMI Spec? Just a thought, as It seems just about everything else has been addressed.

 

[Walkalong]

The firing pin safety block may need adjustment? . Thats my guess after doing some research. http://sigtalk.com/p238-p938-pistol/5258-sig-p938-safety-ccw-questions.html

I agree, after making sure all primers are at the bottom of the primer pocket as Fred pointed out, look for a mechanical issue.

 

[rsrocket1]

I don't have a 238, but looking at the schematic, the fp block is probably what's preventing the hammer from driving the FP into the primer although you said there is the mark of a light strike. That doesn't make sense.

A high seated primer does cause a misfire, but not a light strike. A strike on a high primer will usually be seen as heavily concaved primer instead of a clean FP mark.

You can put a resized empty case in your gun and ride the slide as you are trying to pull the trigger. There may be a point where you will see that the slide is not yet 100% closed but you would be able to release the hammer. See if that's possible. If it happens, look at the primer to see if there is a second light indent.

If that doesn't happen, then ignore this post

 

[RandyP]

I'm thinking there's some crud in the firing pin channel, a burr on the pin itself or the openings or a damaged spring? Can't imagine what else would cause the firing pin to "lock up"?