I'd ultimately STILL believe that there IS a "break in period", even for modern weapons (arguably, moreso now than ever). For most production pistols/firearms, the springs will "work in", the trigger will smooth out, the action will smooth out, etc etc through the first few hundred rounds. But, comprehensive cleaning during this time isn't necessarily critical, other than standard cleaning between sessions to keep things from building up grit in the action and marring moving parts.

Now, on the other hand, there are few things in life that I hate more than spending time cleaning guns, especially scrubbing the bore of a short little barrel. For whatever reason, I can tolerate scrubbing out a rifle MUCH moreso than handguns.

I AM, in this case, a firm believer in properly breaking in barrels. No, this doesn't mean that I detail strip every new gun I own after every shot for the first 100 shots, but YES, it does mean that I do clean the bore and chamber after every shot for the first 50-100 shots. No, it doesn't turn a factory barrel into a "one-patch" clean job, on a few of the rifles that I have replaced barrels for, there is a distinct difference in cleaning time between barrels that were broken in properly, and barrels that weren't. Spending an extra couple minutes/hours pushing a rod through the barrel during the first 100 shots will save you a lot more time cleaning in the next 1,000-10,000rnds, so it's absolutely worth the extra effort for me.