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Discussion Starter #1
I recently tried Titegroup for my .357 and had an interesting occurence. I loaded up 100 rounds of 158 gr LSWC with 4.9 gr of Titegroup, OAL 1.61 and starline brass on my SDB. Hornady lists 5 gr as maximum for this setup but the pressure stated is very low at about 23000cpu. At the range the new load shoots great with little felt recoil and is very accurate and consistant. On my last twenty rounds I am shooting fast doubles when on round 6 I get a boom with about twice the recoil. The cylinder is stuck fast and the primer has backed out of the case. Once I get the gun open its plain to see that round was way overpressure. Luckily no damage to the gun. Since I am using a SDB with its indexing shell plate I can't figure out what happened. I don't see how I double charged it (although I'll concede anything is possible). Is it possible I experienced a detonation? Frankly, I'm skeptical about the concept but I am curious if anyone else has seen something similar. Needless to say, I went back to my old but dirty standby for lead; Unique.
 

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The circumstances do not meet any of the criteria for "detonation" (overbore rifle cartridge, less than 1/2 case of very slow burning powder, and significant throat erosion).

As you surmise, a double charge is about the only possibility. And, as Sherlock said, once you have eliminated the impossible, whatever remains, however unlikely, must be the truth.
 

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Possible you had a squib load on the fifth shot and then the sixth shot blew them both out? This would definately raise the pressures for the sixth round!!
 

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Discussion Starter #4
The other 5 shots hit the targets (steel plates). The Titegroup left the gun cleaner but smoked worse than Unique! I guess I doublecharged it. I've been reloading for several years now and this is the first time. I plan on making sure its the last.
 

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There are several other possibilities:

Debris. Extra stuff in the relatively large .357 case would decrease the available case capacity. One suspect is tumbling media. If you tumble wet brass you can have the media adhere to the inside of the case. If you loaded the case with media stuck inside, the pressure would increase. A moderate .357 load in a case with media that ends up with only .38 Special capacity would be much brisker. Also, a flying spent primer can decrease capacity, but not by as much.

A spilled live primer will cause large amounts of excess pressure if it ends up inside a case.

Are you in the habit of using a case to drop powder, to weight it, then dumping the powder back? Powder adhering in the case when finally loaded burns also. Your 4.9 could have easily become 5.5 or more, if a small amount of powder sticks in the case.

Spiders. If the case was lying on the range for a while, a spider may have built a nest, and the egg mass & webbing would severely decrease case capacity. The decapping pin would create a flash hole for the primer to the powder, but your case capacity would be more like that of a 9mm.

Worst case: You check-weigh powder, use a case with adhering tumbling media, and powder stays in the case. Your 23K load can easily double in pressure, and now you're shooting a 46K load, and flattening the primer.
 

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Another possibility is the bullet on the last shot was not sized properly or at all? The other possibility is maybe the powder measure drop tube section has some debris in it and was bridging a little powder that all dropped into that last casing? I actually had that happen to me before. Found a little wad of cotton like fiber (from the gun powder, I suppose) that would bridge powder and gave me a batch of loads that were mixed hot and light.
 

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Discussion Starter #7
One thing I have observed in the past with Titegroup is it has a tendancy to clump up. In fact I have seen a loosly adhered clump of Titegroup up to 3cm in diameter in the powder can. Maybe this happened in the drop tube? I do not use wet media and alwayes check my brass so an obstruction is unlikely.
 
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