Home Forums Hunting & Shooting Gunsmithing Another "how come" question

Adjunct to my post asking "what is this stuff". These are pics of the bore of that same Ruger SP101. I have a general idea of how barrels are made but don't know the specifics of how Ruger makes their revolver barrels.

So what would cause a pattern like this on all of the lands? It looks like the grooves are cut after the bore is reamed, I can't think of any way for these corrugated lands to come from button rifling or hammer forging around a mandrel. It seems that they ream the bore with a rough and quick first pass and then cut the grooves but I don't know machining processes that well. What does seem obvious is that once the initial hole was drilled in the barrel they never came in with any kind of second pass to clean it up before creating the grooves.

Every land looks like this consistently from the forcing cone (which is a story in itself) to the muzzle. Btw, all of that metal fouling is from five (5) test shots at their factory, I haven't fired this yet and don't intend to.