Before I begin, I absolutely know that they aren't necessary in the .223, however, I find that I have a brick (1000ea) of small rifle magnum primers and given the scarcity of primers right now. The only caliber that I shoot that uses that size is the .223. Considering that I'm shooting a good deal of once fired military brass do you think that a starting load 10% below min. load is unreasonable? Any ideas? By the way these are being used in an AR-15... Probably with Benchmark or Varget, either a 69gr. Sierra BTHP or a 75gr. Berger VLD....

Thnks,

If you have some commercial brass weigh check and compare to the military. if they are close to the same weight then you may not need to reduce the load by 10%.. if you are shooting max loads then reduce and work back up.
I would play on the safe side and when I change any component.. reduce and work up a new load .

Unless your loads are on the edge of excess to begin with, substituting "magnum" primers will have little or no effect on pressures and velocities.