What happens during the "break in" period?

The machine tool marks and burs that were left behind from the manufacturing process are being smoothed-over by the functioning of the firearm. Cartridges or shells are being fed in and out of the chamber area and are also smoothing that surface area as well.

Stiff springs are being used as they are intended to be, but placing pressure on them and then having that pressure relieved.

Many magazine fed pistols follow the browning design where the fit is tight and close among all the pieces. The function of a semi-auto pistol involves friction between the slide, and slide rails or frame. The internal parts are made to fit their home within the gun, but the parts themselves are not a 100% perfect fit like you would get with carefully handcrafted products. Mass produced products are what people usually get (and there is nothing wrong with that). Remember a new gun is fresh including its finish and coating treatments that are applied to the parts of a gun. This all contributes to these parts being worn down to the point that they are not in the way of each other.

A pistol can be reliable before the break-in process, but not as smooth in action. Comparison between a new pistol and a broken in pistol is like comparing how slow a .45 acp bullet travels compared to a 9mm. You can’t outrun either one so what is the real point? New gun, older gun; they will both shoot, and that is the point. Observation of the break-in period is not worth investing too much thought into.

I installed a new barrel on a Ruger 10/22 WRM. It has a breaking in warning. It's accuracy is not as good as the same companies barrel,I installed on a 10/22 Lr. it is 1 hole accuracy at 50 yds. Another warning fired casings not ejecting,getting some of that with both barrels. I have three hi-powered rifles and never encountered any accuracy problems all under 1 inch at 100 yds. and a 300H&H at 200 yds

same as with a motor or anything else that requires break-ins.....the parts wear a little and seat themselves.....microscopic burs and such are worn off.....parts are push against their holding pins so that they get into proper place......ect...

your basically cleaning the grooves between the rifling, allowing better performance with cleaner rifling.

stuff slides into place easier because of the constant shooting