When testing loads for accuracy is it better to use once fired brass or new brass?

Thanks
Jeff

Is it more comfortable to wear old shoes or new ones?

Use brass that has been fired in the chamber of the gun you are testing...

Ditto on what Steve said.
And if possible only neck size your fire-formed brass.

I always buy new brass, load it 5 times then toss it. I full length size every time. So far I have not noticed any difference in accuracy between the new brass and fired brass, whether twice fired or going down the tube on run number 5.

I always try and buy once fired brass the savings are great. I load it as many times as possiable. For the most accurate load use brass that as been fired in your rifle.