Plenty of round nosed and flat nosed bullets have been used in the .30-30 factory cartridges over the years...even full patched round nosed as the below pic shows.
As Abilene indicated, that is because the .30-30 is a bottle neck and the cartridges do not line up in the magazine with the bullet nose resting directly on the primer of the cartridge in front of it. If one lays two .30-30 cartridges on a table, nose to tail, one will see that.
Now straight walled cartridges are another matter entirely, especially if there is recoil. Flat nosed bullets are always best in a tubular magazine. No doubt, many have used round nosed bullets and gotten away with it but it only takes once to ruin your day.
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