Whenever I had a new barrel put on one of my rimfire benchrest rifles (and these are hand lapped custom barrels) the gunsmith always had me shoot a brick of cheap lead std vel ammo thru it to smooth out/fill in any marks left by the chamber reamer. Once that is done, and you start searching for the ammo or lot # that shoots best in your rifle, make sure you clean your barrel between ammo brands. They all use different lube and you want to clean out the brand Xs lube that's coating your barrel before shooting brand Y. Then, it'll take 5-10 shots with the new ammo before their lube fully coats your bore. Once all that is done, then you can shoot for groups that'll be representative of what your rifle/ammo combo is capable of doing.
What I learned after shooting over flags for the 15 years I shot rimfire benchrest is: there is ALWAYS a condition going on that the shooter needs to pay attention to. Sometimes it will seem calm at your bench but your flags will show a wind coming thru between you and the target. I've seen many times where my flags across that 50 yards would show wind going right-left AND left-right. You even have to watch if the sun goes in and out of the clouds. I have seen where on a partly cloudy day where the sun will come out and all of a sudden you will get a thermal that will take your bullet high. Benchrest shooters will tell you that there is always something going on and those who shoot the best pay attention to everything. Any change in condition should result in going to a sighter to see what effect it has on your bullet. It's a fascinating thing trying to dope the conditions. Few master it, but every shooter who really wants to shoot as accurately as possible is smart to get a set of wind flags and try to learn how to shoot the conditions. To do anything else is really just plinking.
