I wonder what the overall length of those nerf bullets are. It looks like they would be long and heavy for caliber. Subsonics maybe? If they are long it will reduce case capacity. Not to mention the problems that would arise from having powder engulfing the fins. If they could produce a projectile which allowed it to spin using the straight rifling this would be a winner.
It's currently only being offered in .300blk so that's why they look so big