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.

I’d send that back to Ruger to be re-barreled, send them a picture and it should do it. It looks like a bottom rifled barrel vs hammer forced which probably wouldn’t come off a mandrel.
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It “looks” like the boring head used was slightly off or chipped which left the thread pattern, then a rifle button when pulled through smoothed out the gloves and left the top of the lands un touched.

Think about pulling a rifling button through the barrel the grooves are pressed in by the button, so it has to be close to the lands diameter, here is didn’t touch the lands very much if at all.

If it’s a 357 - example typically dimensions the lands are 9 thousands taller than the grooves.
The button was probably made for that but if the boring head took a bit too much when it left those groove cuts, then it wouldn’t fill the space in the button for the lands so they were untouched, and you would have what you have in the picture.

Lands diameter = .346
Grooves diameter = .355
Neck diameter = .358

If the Neck cleaned the grooves up, but they are on the lands... there is only .003 difference but that really doesn’t matter if it’s shaving brass like that.

... With manual machinist processes we can stop and looks at the marks the reamer / bore head or whatever , ... is leaving they have a chance to eliminate chatter, or change the tool. But in a production environment the would likely look at the barrels after and make sure they are coming out ok, and this one may have slipped by. If the diameter had been a bit smaller on the bore job... the button may have smoothed it out without any problem.

Thanks, yes, I can visualize that now perfectly. The bore would originally have looked more like screw threads with a sharper or more angular circumferential pattern, then the rifling button ironed those out to be the flattened pattern shown.

Not sure what I'm going to do here. This is the fourth SP101 I've had since June 2020, and this is the second one they've sent to replace two others - both brand new - that they had to scrap due to being "unrepairable". Long thread but the tale of woe is detailed here:

It pains me to say this

I had a Miroku Winchester .22 where the gunsmith's borescope revealed steel chips ironed into the barrel. Likely wasn't cleaned before button rifling.

Stands to reason that a rifling button won't clean up imperfections.

I had a S&W 642 with a forcing cone that looked like the OP's. They rebarreled it.

I have some customers who claim / hate cut rifle barrels because they “foul more” than button rifle barrels
And I have other customers who are exactly the opposite given imperfections on button rifle barrels.

I think the button rifle process can result in more oddball variations than cut... but both shoot well.

When it comes to accuracy, a lot of that comes from controlling (process) accuracy to get consistency so I lean in the direction of cut rifling vs. button.

That looks just a replacement single shot rifle barrel I recently received from the manufacturer except that the reamer marks are in the grooves and faint residual marks on the tops of the lands.

Ruger's attempt at straight line bore knurling.

Total JUNK...& not just the bbl's bore.

Some never learn!

How many chances are you going to give them 10-20.

Sell that POS & move on & Do NOT purchase another POS Ruger...of any vintage.

The newer ruger QC just isn't there. People aren't going to get it, no matter how many times you tell them. I have a buddy that has had 2 RAR centerfire riles with bad bores and chambers.

They don't "bore" barrels, they are gundrilled and reamed. Totally different process. I'd suspect that the reamer picked up a load on a flute or had a chipped flute by how consistent the tooling marks are. Appears the barrel wasn't visually inspected after machining.

Pretty typical when you can sell anything that comes off the line. Just get them out the door. There barrel making has been junk for years.