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Lead-1, The biggest thing that needs to be done to “Break in” a barrel is to get the burrs off the throat. If you’re going to shoot them out, you will be depositing copper down your barrel because the bullets got shredded entering the throat. 100 patches takes off the sharpest edges and gets things headed in the right direction.
It’s better to put some JB bore paste on the patches, or lapping compound if you know how to lap a barrel without ruining it, but even dry patches will have a positive effect.
The whole “shoot 5 and clean 5 times, then 10 and clean ten times” etc etc etc is basically an attempt to work the copper fowling out while your bullets rip off and smooth out all the burrs, but the patching itself is doing a large part of the work as well.
My thought is, patches are for working out bore issues, and bullets are for targets. Using expensive bullets and barrel life to fix a rough bore makes zero sense to me, so my method is to push patches till they feel like silk going down the barrel. I use lapping to accelerate this process, but I hesitate to attempt an instructional post on this because you must have a bore scope and it’s possible to jack up several barrels before you get the feel of it. I do this all the time, and I managed to gain the skill without screwing up anything too badly, but the end result is the same: smooth rifling that doesn’t damage the bullet and therefore doesn’t copper up very quickly, and reduces the swings in precision throughout the barrel’s life.
That’s one of the biggest benefits of a hand lapped barrel: consistent groups throughout the life of the barrel. I don’t personally believe barrel life is extended by lapping or “breaking in” except for the fact that less copper in the barrel will allow you to eek out a few more shots when things are almost toast than the alternative.
A normal barrel being shot with the exact same load will demonstrate fluctuations in precision. We don’t like to think about this grey area, but it’s a fact and it’s not your shooting. Rifle groups will “float” a little as copper and powder fowling build and purge (or get cleaned). A factory rifle will group .85 to 1.25 MOA (ten shot groups). The precision opens and closes as rounds are sent through it, but the average group will be about 1MOA average across the board. A custom rifle with a lapped throat and bore will typically float from .250-.500MOA as the life of the barrel is expunged. Same thing as the factory barrel but much rounder, and much less variation (of course this is AVERAGE and there are exceptions to each of these statements).
So obviously, we are looking for the cleanest most consistent barrel we can get. Comparing a factory barrel to a custom barrel, the differences are subtle. Groove diameter is the same, bore diameter is the same, twist rate is the same, chamber specs are the same, throat is the same, contour is the same, harmonics are similar. The only differences are a stress relieved steel that has more consistent harmonics, and how smooth the inside of it is. THAT’S IT. There’s nothing we can do about the stress relieved steel, but the internal finish is well within your ability to effect!!! So, do what you can to make it smoother. The easiest thing is to push a bunch of patches. After every range session, push patches till they come out lily white like they went in with a TIGHT jag. This strops the steel inside the barrel, and over time, you will have a match barrel.
A friend of Larry Gibson’s taught me this inadvertently because he sent me his Remington model 700 in to be rebarreled. I scope every barrel that comes through the shop, and this one looked like a Krieger inside. There was no bluing left at all, and hardly a tooling mark could be found. Also, the throat was polished beautifully. I looked at the roll mark and it was sure enough a Remington factory barrel. I’d never seen anything like it. I called him up and asked what his cleaning routine was? He told me he simply cleans till patches come out clean. That’s all I needed to hear and that’s why I recommend it.