AFAIK, scope rings will not fit a scope with a bottom rail.
If the scope of your dreams has an integral rail built into it's lower surface, there are two things you can do:
Inquire of the scope maker, what base(s) to obtain/where.
Have a gunsmith measure the rail's clamping width, and whip up a 6" long, one-piece rail that will take the scope on top, and have holes drilled in it that match the pattern of the BAR's scope mount holes. The underside of the custom base will also (of course) be need to contoured to match the slight curve of the BAR's receiver top, side-to-side.
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