Originally Posted by
Sfury
All firearms require so many rounds to go through them to break them in. How many depends on the individual firearm.
My guess is, you won't need to put too many rounds through a .17 HMR before it's good to go.
Not to just poke the bear, I'd tend to disagree, or even believe the opposite to be true.
By breaking in a barrel, we're essentially lapping the bore, which smooths out roughness, and orients the grain structure of the metal together to produce a regular grain structure along the bore.
To lap the bore, the bullet will need speed and pressure.
The .17 HMR cooks around 2500fps, and runs 25kpsi, which is slower than most modern centerfire rifle cartridges, and CONSIDERABLY lower pressure. Compared to a .22lr or .22WMR, the .17 HMR might break in faster, but I don't necessarily predict they'd polish up a bore faster than a faster, higher pressure centerfire round.
Granted, I've only broken in 2 .17HMR's, but honestly, I use a fixed break in procedure anyway, and don't do any measurement except for accuracy checks along the way (not like I'm X-raying the barrel, or using a microscopic borescope after every shot), so it's not like breaking in goes any faster for me using one round or another, but I do know all of my barrels clean up quickly, shoot pretty dang straight, and sure do shine when I stick a borelight in them!