Anyone have experience with RCBS Small Base X-Dies for .223 Rem (5.56x45mm NATO)?

RCBS recommends that cases resized with Small Base dies not be used in bolt action rifles? Can anyone explain why?

I use a Wilson case gauge and find that some cases that pass this gauge test are tight in my Rock River Arms Wilson- barreled Wylde Chamber. Puzzling. Apparently the Wilson case gause is not a sure-fire final gauge.
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