There is all kinds of different opinions about break in. I think most are BS and give the manufacture an excuse if the gun doesn't shoot.
Think about whats happening.
You have a fresh chambered (machined) barrel. There may be tool marks and very sharp edges. When you fire a round, that heat and pressure is going to smooth out and knock some of those sharp edges, and maybe even burrs left over from machining. When this happens, it can rip copper off the jacket and deposit that with the carbon. So the idea is as you break in, you want to keep the copper from getting deposited under the next layer of carbon and building up. As everything smooths out, you want to strip out all the shit so it doesn't just build up. Hard carbon can be difficult to strip without abrasives.
I think the most important thing is to strip all the copper out, so as all the imperfection in the barrel smooth out, and friction reduces (which explains why velocity goes up after 50-200 rounds , which is what we call brake in), you get a barrel that is not imparting uneven forces on the bullet.
I like to shoot about 10, then clean, then shoot 25, and clean every 25 or so shots until break in is complete around 125-150 for MOST barrels. The better the barrel quality (machined true, lapped well and chambered using a good reamer and experienced smith) the easier the brake in is IMO.
For 6 and 6.5mm match guns, I will clean every 250-300 rounds. So that is cleaning every other 1 day or after every 2 day match. Then it takes about 5-10 shots to foul the barrel back in.
The first shot in a freshly cleaned barrel is usually 75-100 fps slower than the rest. the next 5 settle in and I'm back to my usual SD/ES/Velocity.
