Yeah a 5rd accurate, mdt or ai mag with a primal rights kit is about the only option for a purely aics setup.
There are the hawkins hunter mags, the 308/creed magazine holds 4 rounds. They make a 2.5" coal spacer kit, this works for BR variants. Believe this is the same spacer in the hunter 223/arc magazines.
These mags fit mostly flush with their hunter dbm, it protrudes roughly 1/3" below the dbm body. I have a few rifles with the hawkins hunter mag setup and it works well for me, however I have no experience trying thr br variants through them.