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I have a 30x173 casing which I beleive is from the Goalkeeper as it has one crimp and no PGU stenciling. Question is, what caused the (inside out) bump just below the neck?
Has anybody seen this before and what is the cause?
Cornman it is possible that your case was not fired in an actual weapon as such.
It is most likely that it has been fired in a "Proof Housing" where a standard Barrel is fitted to a very heavy steel "pressure Housing" which has a built in Breech block and firing pin.
These "housings" take only individual rounds for testing a rounds velocity or other parameters involving a small risk to firing in a conventional weapon.
Pressure in the cartridge is usually measured by a pressure gauge that has a hole drilled through to the case (but not into the case) and on firing the pressure developed inside the case blows a small disc out of the case wall, when this facility is not used the hole in the housing is "Blanked off" and usually leaves a small "extrusion" on the case wall as in your picture.