RE: Do I get a .243 or a 30/30?
No, the 7mm (.284)bullet is a full millimeter larger in diameter than the .243 (which is 6mm diameter, just to keep the comparison simple). While based on the same case, it's won't chamber in a .243 rifle- would be like trying to push a 7mm diameter rod into a 6mm diameter hole. There are a comple of others with similar cases that likewise won't fit - the .308 Win, and the .260 Rem.
Most, save some foreign and military rifles, are stamped somewhere on the barrel with that specific rifle's chambering (i.e. ".243 Win", ".30-30 Win"). The basic rule is - "Never use anything BUTthat ammunitionin that particular firearm".