The Whisper is based on the .221 Fireball brass, necked up to .30 cal. The BLK is based on the .223, cut down and necked. The BLK has a much thicker neck. The chamber is made to allow for the thicker neck. You will get more neck expansion upon firing with the Whisper and possibly more gas blow by. This will affect performance to some degree. No you will not get "identical" performance with the two, but pretty close
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