If they are using a standard .30 caliber bullet, then its typically because the cannelure is deeper, or the bullet is a lot shorter than the custom "elongated" 300 Blk specific rounds. The 300 Blk specific rounds tend to have a narrower stretched profile out of the case to the tip. This is also to help the round mimic a .223 in the standard AR mags, as the magazine rib can interfere with some rounds and affect feeding.
The lighter 30-caliber bullets designed for .308 and 30-06 are relatively short compared to the 220-240 grain max bullets designed to fit into the 300 blk form factor, they are also a lot more "block" shaped, in comparison with a lot less taper, since its not a concern with those cartridges.
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