Well, I have shot out a barrel in a 300RUM twice. Factory barrel took around 1000 shots. But it wasn't as dramatic as your issue. When you burn out a barrel, it will be gradual. One barrel I had was a Krieger match barrel, and it shot around .5MOA and then got little worse, and then I had flyers. After borescoping, the throat had heat cracks. So it wasn't burned out really but fouled bad after 4 or 5 shots and the flyers would come. The factory barrel that burned out went from 1.1MOA to 1.5MOA around 800 rounds and then went to 2MOA before I rebarreled it. I kept careful records of the bore by measuring the freebore every 100 rounds or so. It was lengthening.

6" groups sounds like a bedding issue or lug contact, something severe.

Make sure the action screws are 40inch-lbs front and around 20 back. The smith may have fited it but did he glass bed it? Two different things. If its movin in the stock, it will never shoot well.

People won't believe it, including myself, but when people grab onto a 300win or weatherby or RUM, it takes special practice to shoot them. I got a win mag when I was young and shot 3-4" groups at 100 yards. I thought the gun was junk. I sold it. Shooting it I would cut my knuckle and forefinger from the recoil. I was shooting it like it was my trusty 270win. The guy who bought it was extremely happy, it was a genuine 1MOA gun. He showed me his groups and I was amazed. Since then and dozens of rifles later, I know that it was me back then. I didn't know how to handle that gun. I since then have shot all kinds of >30 ft-lb recoil guns and seem to do much better. Before shooting a 338RUM or simular, I need to be in the mood. Calm and light on the coffee that morning.