Don't be a gmor.
That’s great can I use that?
To the OPs question. I do not break a barrel in but rather more technicially speaking I build it up. With my brand new GA Presicsion FBI HRT rifle I followed in instructions presented in the Sniper 101 videos series. Basically what I am doing is building up a solid bearing of copper in the barrel so the the bullet is gliding on as much copper as possible during its trip down the tube. What I noticed was that my MV increased and my groups decreased as the copper bearing surface was built. I did this with standard ammo off the shelf while getting my scope in the general vicinity of the bulls eye. After about 60 rounds I had gained about 50 FPS and the size of the groups had halved. I then switched over to the ammo I was intending to shoot for the LD course I was going to attend. I noticed right away that while my scope was still close, the groups opened back up but as I continued to shoot the groups closed back up after about 15 rounds. Different manufacture of bullets so somewhat of a different recipie with the copper jacket but as the bearing surface built to match the copper I was sending down, the groups got better all the way down to less than 1/4 inch at 100 yards.
To this day I have yet to clean the copper out of that rifle and with almost 1000 rounds down through it I am still getting the same result. 8541 Tactical (former Marine Sniper and current LEO sniper) did a video of this a couple of years ago switching between Hornady and GMM. You can see an immediate increase in the group size from the same rifle but after 10-15 rounds it goes back to the size of the group with the previous ammo. I did however take another barrel that I had been using shot a test group, stripped the copper out and started shooting groups again with it. Same result and once again after about a dozen rounds the group came right back to where it was before the stripping.
Now if you want to call this barrel “break in” what ever but it isnt. First there is very little chance that anything other than erosion is happening when you shoot quality ammo down a quality barrel. Clean any preservative from it, shoot it a few times and clean the powder fowling but leaving the copper bearing surface will make better groups and higher MVs.