There are a lot of variables that can effect velocity, and different barrels can contribute to this greatly,
In my opinion, if it takes 100+rounds for a barrel to settle in it was not broke in properly. Some barrels take longer to break in, especially factory barrels. They don't have the bore finish that custom barrels do
and I have seem them take up to 60 or 70 rounds to settle in and some realy never did, but they shot as good as they ever would.
Most High quality barrels will break in with anywhere from 10 to 30 rounds through them if the clean and shoot method is used and will settle in to an average velocity. The quality of the ammo or reloads
has a lot to do with how fast you start getting stable velocities also.
so I use the same load to reach this point because it shows the difference from shot to shot. I start load development only after the barrel settles down and consistent velocities and SDs for that load are achieved.
This can sometimes be a very boring process but it is well worth the effort. And even though the load may not produce the desired results, It is an apples to apples comparison to show the state of the barrels brake in and can tell you when you should/can start load development. the fact that we start loads on the low side and work up has a lot to do with increase velocity as we go but I have found that the barrel will reach consistent velocities much sooner if the load doesn't change.
A while back I did a velocity test while breaking a barrel in to see the effects on velocity. and after the test, the velocities have not changed enough to consider it significant except when I increase the powder charge or burn rate. (Pressure) I have verified this by using the leftover break in loads that at some point are used for fouling shots before a hunt to check on the barrel condition. A good bore scope will give you an indication of the bore condition, but a velocity test using the same load as you started with will give you proof.
Here is the results of the velocity test during brake in.
https://www.longrangehunting.com/threads/new-barrel-break-in-and-cleaning-methods.160450/page-2
Just My opinion based on my experience with new barrels.
J E CUSTOM