What that means is if you send an NK SB2 frame in for an accessory barrel, they'll only fit it with 357, 44 mags and shotgun barrels, if you needed a replacement barrel for the original defective/damaged/worn out 30-30 barrel, they'd only fit it with the original chambering, they won't fit any other barrels to it other than those stated. I don't know if they'd fit another 30-30 accessory barrel to it, you could call and ask if that's what you'd like to have done.
Tim