They fit the barrel to the receiver. The NEF/H&R policy is that you only use barrels on your receiver that have been fitted at the factory. To be safe, if you are inexperienced in this area, it is best to follow the NEF/H&R policy. In the event you would aquire a barrel from e bay or somewhere else, you could take it to a gunsmith and have it fitted. The factory will only fit barrels they sell.
NEVER PUT A RIFLE BARREL ON A SHOTGUN RECEIVER. They may looke the same and it may fit and lock up fine, but the shotgun receivers are not made to withstand the higher pressures of rifle created by rifles. You may, however, have a shotgun barrel fitted to a rifle receiver.
It is not a dumb question. I am glad you asked. Have fun shooting your 243 and 270.
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