Gap between bolt lugs and top round in mag when bolt held fully rearward
I'm curious because this measurement governs how wide the cycle timing is. AK measures 1.688". AR15 is 0.688". Bren 2 is 0.813". What does the XCR measure here? I'm curious what Alex Robinson went with.
A wider gap here allows a rifle to be more overgassed, or function with wider pressure curve variances in ammo, or have magazine feeding slow down more (using a drum mag for instance) and still have everything remain within the timing window for function. A narrower gap saves weight and length.