Any base diameters from .452" to .460" are within SAAMI .303 specs. (.30/40 spec is .001" greater.) What size comes out on a specific production run simply depends on the finish production die's history. New die: small base - worn die: large base. Trying to predict what you'll find in the next batch you buy based on the kind of small sample most of us enthusiasts have available is an exercise in futility.

303carbine, CIP maximum cartridge base diameter (P1) for .303 Britishicon is 11.68mm. With a tolerance of -0.2mm giving 11.48mm minimum, your figures of 11.38 to 11.43mm seem low. Is it possible you aren't measuring immediately adjacent to the rim or your caliper needs calibration?

Attachment 64101