Re: once fired or virgin brass
This is from my 30+ years of handloading. I have found that mixing makes of cases does make a difference in point of impact as well as group size.
If you want the best accuracy you can get, and that is why most people handload, you need to keep everything as close to the same as possible. Same lot numbers of everything etc.
I ran into this problem once. I had a new out of the box Rem. 700 stainless in 7mm Rem mag. Before I shoot a Rem. 700 I always free float the barrel and adjust the trigger to 2 1/2 pounds. I bought 100 new Remington cases and used the same lot numbers of powder primers and bullets. I FL sized my cases and started working up a load. I tried a bunch of loads and could not get that rifle to shoot better than 1 1/2 inch groups at 100 yards. I was shooting the new cases all the time. I decided to try once fired cases with the best load. I shot one round and hit the target. I fired the second round and looking through the spotting scope I still could only see one hole in the target. I thought great now I can't even hit the target. I fired the third shot and still only one hole in the target. I decided to walk down to the target and see what was going on. I found that all three bullets had went into almost the same hole. I went back and proceeded to put the last two rounds I had loaded up into virtually the same hole. I found that you could fill a part sized just bumping the shoulder, previously fired in that rifle fill the case up to where the neck started with AA3100 and cram any bullet from 139 gr Hornady up to 160 gr Sierra and they would go into almost the same hole at 100 yards. One of the most accurate rifles out of the box I have ever had. Moral of the story. You will never know unless you try it.