The bren was innitially chambered for the then current british ammunition which was .303" not 7.62mm. The only bren guns to fire the 7.62 NATO spec ammunition were modified mark 3 and above units and these were not in use until post world war two. The russian ammunition at the time was 7.62mm. Note the progression of ammunition types right up to 76.2mm for the t-34 tanks equiped with what was called the t34/76.
It would not have been logical for the russians to use the 7.92mm calibre used by most german weapons for logistical reasons.
As for experience, my father used the bren gun as his personal weapon during national service and they had a mixed bag of .303" and 7.62mm because the british army were then rearming with the SLR 'Belgian FN' and the ammunition was being standardised. I also had a bren gun as my personal weapon while training recruits and we had a dinosaur of a .303" chambered bren, and no ammunition to fire from it. Your information is flawed sir.