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Unsupported Chambers

Posted Feb 6th, 2018

A chamber provides full cartridge case support only if the case is supported down to the level of the INSIDE BOTTOM of the case, which typically is about 0.05-inch (one-twentieth inch) above the start of the case extraction groove outside. Thus any chamber that provides case support all the way down to the extraction groove more than fully supports the case.

Unfortunately, typically there is an area above the feedramp that must be relleved some to allow the round to feed reliably into the chamber. That is where case blowout may occur.

Only tappered-case or bottle-neck cases make it easy to design chambers with full case support above the feedramp. For Glocks, only the chambers for the bottle-neck .357SIG provide both tight chambers and full cartridge case support even above the feedramp in OEM barrels. (Thus, it is incorrect to say the all auto-pistol barrels lack case support…the .357SIG OEM barrels are perfect examples of full support…made possible ONLY by the bottle-neck case.)

It is true that there has in the past been need for Glock to improve the case support for other calibers…the .40SW models in particular. By the mid-2000s those changes had been made. Below is a photo showing the support provided by the worst-case to best-case Glock OEM barrels:

.40SW barrel
Left – Glock OEM 1997 .40SW barrel – Worst-case marginal support
Middle – Glock OEM 2012 .40SW barrel – Much improved support
Right – Glock OEM 2012 .357SIG barrel – Best-case full support

The differences in exposed case area are obvious from the photo.