on one hand... yes. Sure you can see the impact of internal stresses (residual from the button swaging that cuts the rifling) opening the bore as material is removed from the tube's OD. This is not particularly surprising. I've also read that it's not an issue with single-point cut-rifled, which also would not be surprising if you think about the manufacturing stresses. On the other hand, this video doesn't prove anything relating to accuracy, but rather it proves what a good metallurgist or material scientist would have told you; things with residual stresses sometimes warp when you cut them. What it doesn't directly prove is that it's definitely detrimental to accuracy. I'm sure the muzzle bore belling outward isn't helping anything either, but when you can go buy button rifled threaded barrels from any number of premium outfits (proof, lilja, shilen, criterion, mcgowan, bergara, et al) that shoot sub MOA all day... well maybe it doesn't really matter in the end. What you have here is actually only evidence (not necessarily proof). To become proof you have to establish a cause-effect relationship of a documented observance. I think you'd need to do a comprehensive test with several dozen barrels of various types of manufacture to see if this evidence you posted actually contributes to the stated conclusion that 5/8-24 threading is detrimental. For example, take several barrels with target muzzles and crowns and then gage them and then shoot them. Thread 3/4" leaving the crown untouched, gage them, and then shoot. Thread 5/8" leaving the crown untouched, gage them, and then shoot. Thread 9/16" leaving the crown untouched, gage them, and then shoot. Then look at the results comparing each barrel to it's previous iterations. It should be pretty controlled because you've got the same chamber, barrel, crown, and load.

Very interesting! Many years ago I was told by a salesman at Knights Armament Company that they do not thread the muzzles of their SR-25 barrels because it "messes up the crown of the barrel." He didn't explain why, but now I know. I also had a 6.5 Grendel rifle from Alexander Arms which had a 3/4 x28 threaded muzzle which I thought was very odd, but now I know why.

I stumbled across this video looking for information on threading my current 223 Barrel, and this is actually pretty interesting makes me rethink what I'll thread mine as. Thanks for the info

Thank you for that insightful and informative demonstration.

Nice video! We get these questions all the time. I say run the largest thread diameter that is possible. Also what you are seeing is common with button rifled barrels. The machine work is relieving any residual stress in the barrel and it causes the bore to open up/go sour. Frank Bartlein Barrels

Were the barrels checked before threading?

The owner of bartlein barrels is adamant about using only 3/4 diameter threads on precision rifles.

Interesting

May be a silly question. What if you thread leaving the gauge pin in. Would that help prevent the swelling and keep the bore to size may be a little hard removing the pin but ????

OK got a question, I’m getting old, I have a 300 win mag I love. I’ve wanted to have my barrel threaded for a suppressor, are you saying I’m going to lose my crown? The firearm is a Weatherby Accumark USA made, I’ve been shooting sub MOA for some years, Tub final finished barrel years ago. What would you recommend, thanks

Is it possible to cut the rifling after cutting the 5/8 x 24 thread?

Are the gauge pins tapered ?