Ammo types and the semi auto Bren

[AmEngRifles]

I wanted to start a conversation, and possibly a data base, no matter how limited, associated with the semi auto Bren. The purpose is to possibly come to some conclusion on what types ammo work best in your particular gun.

As I have several semi auto Brens, I will talk about this from the perspective of each individual gun and build at a time.

I had my Project Guns Bren MkIII at the range yesterday with some British, WWII Mk VII out of one of those typical wooden cases of 288 rounds. It was packed in individual 32 round boxes. I am not clear where this ammo is produced, but the headstamp on the case has both "GB1944" and "VII". I figure this is corrosive ammo, so clean accordingly after use.

It ran fine as I used my MkIII custom barrel on setting 3 on the gas regulator. Decided to try gas setting "2". Immediate case head separation, right about the rim, so the chamber pretty much has a full brass case stuck in the barrel. BTW, my barrel has good headspace.

As I policed the previous brass cases that had fired, I noticed that all the necks were displaying cracks and splits in the neck. Nothing from the shoulder down. So my question is, does this old ammo become more brittle with time? Is there something in the composition of the powder or primer that leads to brittle brass?

I had just been firing this rifle with HXP from 1975 and that ammo worked beautifully. The cases all come out nice and I am able to actually reload those. No apparent brittleness.

I am also wondering how changing the gas setting may have made the cases extract slower (?), leading to the case separation. Don't see how less gas could make the case extraction quicker? That may not be what happened at all, but wondering why all was good at setting 3, then went sour with setting 2?

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