There seems to be some confusion here about primer sizes and standard vs magnum primers.
There are small and large rifle, and small and large pistol. The external dimensions of the small primers are identical (sp & sr), as are the large (lp & lr). The biggest difference in those are cup material thickness/hardness and charge strength. Within each size, say large rifle or small rifle, there are standard and "magnum" options. Again, these are dimensionally identical, with differences in cup thickness and charge potency.
If I understand you, Buffalo Bore is using Starline Small Primer brass, so you should just be able to use a small rifle primer, and can use a magnum small rifle primer for higher pressure resistance. CCI 450 should be the small rifle magnum primer.
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