7mmBR is based on 6mm BR which evolved from the .308 x 1.5 Barnes which was based on .308 Winchester. Shoulder diameter for the 7mm BR is .4598" and the shoulder diameter for 7 x 57 is smaller, .430".
Now, the 7mm BR shoulder is going to be less than 1.5" from the case base.....something like 1-1.2" from the base. 7 x 57 case is .473" at base. IF the 7x 57 taper is such that case diameter is greater than .4598" 1-1.2" from the rim then a rechamber might work. I'll mike a 7 x57 case later today.
My neck sized 7 x 57 reloads measure .450" one inch up from the base. Given the 7mm BR is .4598" diameter that far from the base, it doesn't look like a 7mm BR rechamber to 7 x 57 would work without setting the barrel back.
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