age of the rifle and value
The question is moot, here's why: First of all, the replacement must supply the correct voltage. This cannot change. You always replace a 12V supply with a 12V supply. If the replacement supply has the correct amperage, it will also have the correct volt-amperes, since (by our definition) the voltage is the same. Say the old supply was 12V, 2A. This is a 24VA supply (12 X 2 = 24). The new supply also must be 12V, so which is more important, amperes, or VA? As you can see, if we make sure the new supply is 2A, it will also mean it supplies 24VA (12 X 2 = 24). If we make sure the new supply is 24 VA it will obviously supply 2A (24/12 = 2). So, assuming the voltage stays the same, matching either VA or amperes automatically means the other value is correct.
The output voltage of a transformer is not connected to the maximum rated current. You state that the transformer is rated at 12-0-12 at a current of 1A, therefore the maximum (rated) current on the secondary would be 1A. Normally, transformers are rated in VA (volt amps) which is simply voltage x current, the voltage across the two 12v terminals would be 12+12 volts, making the rating of the transformer 24VA.