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Waiting on my third barrel for my .357 Steel Big Boy the original barrel had so much chatter in it that accuracy went down hill after about 10 rounds and would shoot patterns instead of groups after that. When I would clean it I would initially get hunks of copper. The bore on the replacement barrel looked good with no chater then at the last half inch or so things went south. It not only had the strange marks but the rifling actually went to a tighter twist.

Any one have a guess what could have caused this.

821787
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It could be caused by a full rifling tool (reamer?), from what I've been told.

My 1980 Glenfield 30A had those chatter marks in the bore, and I had the same problems you had with accuracy and copper fowling. I had JES re-bored it to 375 Winchester and now it's a tack driver. So that proves it wasn't the quality of the barrel material.
 
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IMO what I see is grooves or tool marks perpendicular to the rifling. Being a retired toolmaker I would have to say that is caused by a "bug" as some old time machinists called it. A bug is a chip the reamer cut that that couldn't get out of the way and is stuck or fused to the reamer. As @ballistic04 mentioned a "full reamer" ( one that has not been cleared or cleaned of chips frequently enough could cause that) Being barrel material it can cut into the barrel as it is turned causing that groove as I'll call it.

The bore AND the grooves in the picture look very rough to me also.
 
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