The ejector doesn't strike the cartridge cap. It strikes and deforms the cartridge case JUST above the primer pocket. This is just sufficient to firmly hold the primer in place during the harsh EJECTION process. This is not the ony thing that prevents 'caps-out' as the yrials report suggests.
The deformation of the primer by the heavy oval striker or firing pin also very effectively ensures that its ovality spreads - displaces the primer material towards the top and bottom edges of the pocket. The primer is well sealed.
To be honest, in my dealings with Bren guns, Mk2's in the UK and Mk3's in Malaya, caps-out was never a problem of any consequence to us. Clever chaps those Czech designers.......