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Old December 28, 2019, 09:02 AM   #180
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Join Date: June 17, 2010
Location: Virginia
Posts: 5,681

I would actually expect case stretch, datum-to-boltface, in high-pressure cartridges like the 350/5.56: firing pin strike drives the case inward to max seating, pressure glues the forward/thinner sidewalls, and the case head stretches back to boltface.

That the stretch and/or case movement back to the bolt could then produce "statistically-feasible" uncovering of a weak enough portion of the case web to allow gas breakthrough into the action... is troubling to say the least.

FWIW: I ran QL for the 180 Speer at 42ksi for both Lil'Gun (incipient failure/"belt" produced) and Norma-200 (totally normal). Lil'Gun's rise-time is peaks earlier relative to bullet movement, and remaining pressure as the bullet moves past the port is lower -- yet still that powder produced the anomaly.

Truth in lending: The Lil'Gun anomaly occurred using the LEE 358-200 cast at 42ksi indicated -- and measured velocity was waaaay outta whack* to where I couldn't even jigger Lil'Gun's burn parameters to match it at all -- but the incipient failure mode was the same as yours.

... bothersome that it could be at luck-of-the-draw bolt/chamber/case design (stacking) tolerance levels..... Yeah, cast seals better/a bit higher pressure resultant, but not totally-off-the-charts higher* as indicated by the LABRADAR on two successive shots -- only one case of which "belted" on me.

* I still can't explain why I (and you) saw those velocity/pressure spikes

Last edited by mehavey; December 28, 2019 at 09:12 AM.
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