What caused this squib?

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I suspect your contamination is "a tablespoon of brass polish". If you put too much brass polish in your media, clumps will form and stay in the cases even after full length sizing and de-priming, even blocking flash holes. It's great to read instructions, which I admit I never did on the brass additive cleaner bottles. Three different brands of brass cleaner additives say to initially add a Tablespoon or more per pound corn cob media. Then next time only add a cap full of 1/2 teaspoonful when the media loses it cleaning effectiveness. Problem is corn cob or walnut media suppliers tend to add additional cleaner additives to new media. After adding the additive, one is suppose to run the tumbler for 3 to 4 minutes BEFORE adding the brass. Now how many people actually follow those directions?
 
I've also had corncob media hang in the flash hole even through the decapping process. I'm betting on a piece of crusty corncob/polish hanging there on you and interfering with the firing ignition process.
 
The brass polish theory makes sense to me. The first 31 reloads were done with brass that was cleaned without polish. All fired fine. The polish arrived later and was used for the batch giving me problems. I'll dump the old media, use new, skip the polish and start over with "new" brass. Edit: Another possible cause may have been dry tumbling my brass in my 35 degree garage for two hours because of the noise and dust. I brought it in to my 75-78 degree basement to reload which might have caused condensation in the case resulting in damp powder. I remember the brass feeling very cold. I'll add that to my "never do again" list.
 
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Just check inside all the cases, if you use too much polish it can get stuck inside sometimes, toss some of the media and dilute what's leftover if that is happening. Toss any cases that are too crudded up to fool with.
 
The brass polish theory makes sense to me. The first 31 reloads were done with brass that was cleaned without polish. All fired fine. The polish arrived later and was used for the batch giving me problems. I'll dump the old media, use new, skip the polish and start over with "new" brass.
There ya go. Keep things simple. Less opportunity for Murphy to call The Law on ya.
Ya gotta think, " what price polished brass? "
 
I reloaded 12 test rounds today trying to eliminate any potential issues like the squib I had. I used dry corn cob tumbled brass with no polish and zeroed the scale. I used the primer pocket cleaner on all cases. Weighed several powder dumps on beam scale. All 4.0 grains HP-38. I loaded four cases directly from the scale and marked them. I used the Auto Drum on the remaining eight. This problem been on my mind since it happened and I'm determined to get it figured out.
 
stonebuster, you do not have to give up on using the polish with the dry media. I use it with both crushed corn cob and crushed walnut. I use NuFinish and have had no problems after I learned to run the tumbler for 10 - 15 minutes after adding the polish and before adding the brass. Before I learned to do this, the polish would stick in clumps to the brass and cause trouble. If the clumps of polish are coated with the media they will not cause trouble.
 
Sounds to me like no powder, just primer.... unburned powder could have been there from previous rounds.
I visually checked every case for powder before seating the bullets. There was quite a bit of unburned powder left on the cylinder face from the squib as I mentioned in original post.
 
My best guess is not enough crimp, meaning not enough pressure built up to completely ignite the powder, leading to your squib. Post a pic of your finished rounds and somebody with more knowledge than me will show up with an answer.

chris
Maybe I should try a couple with a tighter crimp. I was trying not to over crimp the plated bullet and was under the impression a light crimp was sufficient. I suppose the worst would be poor accuracy/ fouling if I overcrimped the plating? But there's something amiss with lot unburned powder left from a squib and your "best guess" is worth a try. There were a few others that sounded weak also and I very much doubt I had powder dumps that varied that much because I was using an abundance of caution. Is there any reason I can't go back and put a tighter crimp on some from the bad batch? How's the crimp(Lee fcd) look in my pic?
 
Ever check any loads powder forward against the bullet? ( I no longer own 5 of those pistols listed)
Load # 132 .357 Mag.JPG
 

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Rereading your original post I see several things:

1) "No bang, just Poof. There was a good deal of un-burned powder(flakes, like right out of jug) left in the cylinder chamber, cylinder face and forcing cone."

Simple logic says the primer ignited. If not the bullet would not have moved and you wouldn't have had powder everywhere. Possibility of the primer being faulty is very slim, although it is possible. You can try a different brand primer if you can find them.

2) "I'm absolutely certain there was powder in the case because I've pulled every case out of the shell holder to visually confirm powder was dumped before seating the bullets."

Good safety check. Doesn't mean you didn't have a light or heavy drop in some cases. Personally I weigh every charge, but I am OCD about some things. A light powder charge may have caused a squib, but most likely would not have left a "good deal of un-burned powder" as you described.

3) "I've been placing the primers in the primer arm with my fingers rather than the feeder. I've read that can lead to contaminating the primers."

This is possible but unlikely. I wear nitrile gloves while reloading, to keep my hands clean and not contaminate any components, and I also place every primer in the primer arm. Primers are pretty tough, and last a long time according to my research.

4) "Last night I decided to see how these rounds chambered in the cylinder and they were slightly difficult to load. The case lip where it meets the bullet seemed to need more crimp (lee fcd) so I ran the through the crimp die again after adjusting it tighter."

"Slightly difficult to load" indicates to me that you may need more crimp, OR that your cylinders may be crudded up and need a good cleaning. I have no experience with the Lee FCD so I can't/won't comment on that. I roll crimp all my 38spcl and 357mag rounds into the cannelure.

I still believe, although I may be wrong, that you had a pressure issue (not enough).

I said you may need more crimp, but you may need more neck tension also. Your sizing die may not be screwed down enough combined with not enough crimp, leading to your bullet starting to move out of the brass before the powder ignites completely. In basic terms, enough pressure to start the bullet moving, but not enough to push it completely out of the barrel or get a complete burn of the powder, leaving un-burned powder as you described.

FWIW, for 38spc I run 3.9gr of W-231 (same as HP-38) under a 158gr sjsp (semi jacketed soft point) through a 6" barrel and have no issues. Your bullet is plated vs. my jacketed, but I don't feel your load is too light.

chris
 
The consensus seems to indicate the 4.0gr HP-38 is not my problem. The unburned powder makes the bullet moving out of the case before the powder fully burns theory make sense. I heavy crimped 12 left from the bad batch and medium crimped another 12 to eliminate light crimping being the problem. I try to get to the range tomorrow.
 
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