In custom barrels, I don't know that I'm a believer in a break in period...in production rifle barrels...that's another story.

When we're talking about barrels that get put out in the thousands from an assy. line, you KNOW that bore is going to be rough. A break in period helps smooth things out in a more regular manner.

Here's one example....have you ever shot a cap and ball revolver? For the first hundred rounds or so they sound like pop guns, or like a shotgun squib load, just a "PUUUNF" and you can't hit the broad side of a barn with them, after a hundred rounds or so, they start "CRAAACKING" and you get higher velocities and more energy out of the same loads, and they finally start shooting well....Old buckskinners will tell you that the "oil is out of the bbl" (not gun oil, but the bluing) when this happens, others say it's because the lead from your slugs has finally fouled up the barrel enough to seal properly...One way or the other, something is happening in that barrel.

One thing that I really think is the key...When you first buy a rifle, the grain structure of the steel is arranged rather randomly, or more likely in a spiral pattern consistent with the tool that cut the bore....The point of breaking in is to let all of the grain structures be REGULARLY patterned and realligned by the bullets going down the bore. In order to guarantee consistent contact, you need to make sure there isn't any fouling between your bore and bullet...getting things all lined up reduces the bbl roughness which reduces the stress put on the bbl by each shot (a little bit at least) so the life span of your bbl would be increased.

I've had several rifles that I almost sold because they shot terribly out of the box...after a hundred rounds or so, they started shooting very well. I've also had a Rem 700 and a Ruger 77 that I did NOT break in that NEVER started shooting well (Ruger and Rem replaced them luckily). I didn't even think to break in my first Ruger Revolver, I got pretty good groups with it anyway, then I bought a matching partner to it and broke it in...it shot about on par with the other one when I first started, now it consistently shoots groups at least an inch smaller than the other.

Now, can I honestly say that the break in had something to do with it? Probably not, unless I had two EXACTLY IDENTICAL guns and broke one in and just shot the other, I'd never be able to say for sure....but based on my experiences, yes, I'll continue to break in bbls. I do honestly think that bbl break-in periods at LEAST help your bore/bullet seal be more regular and will in fact have an effect on the accuracy and life span of your gun.

I don't believe, however, that you'll HURT your rifle by not breaking it in, and even if you don't, it should eventually reach the same point, it'll just take longer to get there. You're not going to make your brand new rifle that shoots 1.5MOA shoot 3MOA because you don't break it in, and break in isn't going to take a 5MOA rifle and make it a .5MOA rifle, but I do believe it has some effect.